[m-rev.] for review: update backends page on website

[Julien Fischer]

For review by anyone.

Note to reviewers: the content for the remaining backends has not changed,
although I have reformatted the source file.

---------------------

Update the backends page on the website

The changes are:

- delete references to Erlang, bytecode, assembler and old .NET backends.
- delete the pull quote; it no longer conveys any useful information.
- do not distinguish between "mature" and "beta" quality backends.
- use flexbox based layout.

about/include/backends.inc:
    Make the above changes.

css/common.css:
    Redo the two column layout we used on the backends page using
    flexbox.

Julien.

diff --git a/about/include/backends.inc b/about/include/backends.inc
index eba4864..f23995f 100644
--- a/about/include/backends.inc
+++ b/about/include/backends.inc
@@ -1,155 +1,81 @@
  <h2>About Mercury<span class="submenutitle">/ back-ends </span></h2>
  <p>
-The Mercury implementation compiles to a wide variety of target
-languages on a wide variety of platforms.
-Several of these back-ends are quite mature, other have been
-released but are still immature, while others are under development.
+The Mercury implementation compiles to a wide variety of target languages on a
+wide variety of platforms.
  </p>

-<p class="backendssubsec">
-Mature
-</p>
-
-<div style="float:left; max-width: 420px; margin-right: 50px;">
-<h3>Low level C.</h3>
-<p>
-	Our original back-end.  This one compiles
-	Mercury source to low-level C code.  It works by converting
-	Mercury source into code for an abstract machine, and then
-	converting the abstract machine instructions into C code
-	with macros for the abstract machine registers and instructions.
-	We use C's conditional compilation (<code>#ifdef</code>)
-	to define these macros as either standard ANSI/ISO C code, or as
-	GNU C code that makes uses of GNU C extensions such as
-	global register variables, computed gotos, and inline assembler.
-	</p><p>
-	For more information on the low-level C back-end, see the
-	<a href="<?php echo $root?>/documentation/papers.html">papers</a>
-	section of the web site, in particular the papers titled "The
-	execution algorithm of Mercury: an efficient purely declarative
-	logic programming language", "Code generation for Mercury",
-	and "Compiling logic programs to C using GNU C as a portable
-	assembler".
-</p>
+<div class="backend-container">
+<div class="backend-item">
+  <h3>Low level C.</h3>
+  <p>
+  Our original back-end.
+  This one compiles Mercury source to low-level C code.
+  It works by converting Mercury source into code for an abstract machine, and
+  then converting the abstract machine instructions into C code with macros for
+  the abstract machine registers and instructions.
+  We use C's conditional compilation (<code>#ifdef</code>) to define these
+  macros as either standard ANSI/ISO C code, or as GNU C code that makes uses of
+  GNU C extensions such as global register variables, computed gotos, and inline
+  assembler.
+  </p>
+  <p>
+  For more information on the low-level C back-end, see the
+  <a href="<?php echo $root?>/documentation/papers.html">papers</a>
+  section of the web site, in particular the papers titled "The
+  execution algorithm of Mercury: an efficient purely declarative
+  logic programming language", "Code generation for Mercury",
+  and "Compiling logic programs to C using GNU C as a portable
+  assembler".
+  </p>
  </div>
-<div style="float: left; max-width: 420px;">
-<h3>High level C.</h3>
-	<p>
-	This back-end compiles Mercury code to fairly high-level C
-	code, more like the kind that a C programmer might write.
-	</p>
-	<p>
-	For more information on the high-level C back-end see the
-	<a href="<?php echo $root?>/documentation/papers.html">papers</a>
-	section of the web site, in particular the paper titled
-	"Compiling Mercury to high-level C code".
-	</p>

-<div class="quote" style="max-width: 350px; float: left; margin: 20px 20px 20px 20px; border: 1px solid white;" align="left">
-<h2 style="font-size: 19px;color: #2e3192; font-style: italic;">
-“Several of these back-ends are quite mature, other have been released but are still immature, while others are under development.”
-</h2>
-</div>
+<div class="backend-item">
+  <h3>High level C.</h3>
+  <p>
+  This back-end compiles Mercury code to fairly high-level C code, more like
+  the kind that a C programmer might write.
+  </p>
+  <p>
+  For more information on the high-level C back-end see the
+  <a href="<?php echo $root?>/documentation/papers.html">papers</a>
+  section of the web site, in particular the paper titled
+  "Compiling Mercury to high-level C code".
+  </p>
  </div>

-<div style="clear: left;"></div>
-<p class="backendssubsec">
-Beta-release quality
-</p>
-<div style="float: left; max-width: 260px; margin-right: 50px; border: 0px solid black">
-	<h3>Java.</h3>
-	<p>
-	This back-end compiles Mercury to Java.
-	</p>
-	<p>
-	The compiler support is complete,
-	it includes good support for interfacing with Java
-	(see the language reference manual for details),
-	and the standard library is fairly reasonable.
-	There is support for Mercury-level debugging, but there is
-	not yet support for Mercury-level profiling.
-	</p><p>
-	For a guide to installing and using the Java grade, refer to
-	README.Java and the User's Guide. You will need Mercury release 10.04
-	or later.
-	</p>
-</div>
-<div style="float: left; max-width: 260px; margin-right: 50px; border: 0px solid black">
-	<h3>C#.</h3>
-	<p>
-	This back-end compiles Mercury to C#.
-	</p>
-	<p>
-	The compiler support is complete,
-	it includes good support for interfacing with C#
-	(see the language reference manual for details),
-	and the standard library is fairly reasonable.
-	There is support for Mercury-level debugging, but there is
-	not yet support for Mercury-level profiling.
-	</p>
-	<p>
-	For a guide to installing and using the C# grade, refer to
-	README.CSharp and the User's Guide. You will need Mercury release 11.01
-	or later.
-	</p>
-	<div style="margin-bottom: 50px;"></div>
+<div class="backend-item">
+  <h3>Java.</h3>
+  <p>
+  This back-end compiles Mercury to Java.
+  </p>
+  <p>
+  The compiler support is complete, it includes good support for interfacing
+  with Java (see the language reference manual for details), and the standard
+  library is fairly reasonable.
+  There is support for Mercury-level debugging, but there is not yet support
+  for Mercury-level profiling.
+  </p>
+  <p>
+  For a guide to installing and using the Java grade, refer to README.Java and
+  the User's Guide. You will need Mercury release 10.04 or later.
+  </p>
  </div>
-<div style="float: left; max-width: 260px; border: 0px solid black">
-    <h3>Erlang.</h3>
-	<p>
-	This back-end compiles Mercury to Erlang.
-	</p>
-	<p>
-	The compiler support is in good shape,
-	it includes good support for interfacing with Erlang
-	(see the language reference manual for details),
-	and the standard library is fairly reasonable.
-	There is support for Mercury-level debugging, but there is not
-	yet support for Mercury-level profiling.
-	</p>
-	<p>
-	For a guide to installing and using the Erlang grade, refer to
-	README.Erlang and the User's Guide. You will need Mercury release 10.04
-	or later.
-	</p>
-</div>
-
-
-<div style="clear: left;"></div>
-<p class="backendssubsec">
-Dormant, not currently under development
-</p>
-<div style="float: left; max-width: 260px; margin-right: 50px;">
-	<h3><a name="bytecode">Mercury byte-code.</a></h3>
-	<p>
-	We have our own bytecode format,
-	a back-end which generates it, and an interpreter
-	which runs it.  This is designed to interoperate
-	with our low-level C back-end.  The main aim here
-	is to get better turn-around time for debugging.
-	</p>
-	<p>
-	This one still needs quite a bit of work to integrate it
-	with the rest of the Mercury implementation.
-	Read the source.
-	</p>

+<div class="backend-item">
+  <h3>C#.</h3>
+  <p>
+  This back-end compiles Mercury to C#.
+  </p>
+  <p>
+  The compiler support is complete, it includes good support for interfacing
+  with C# (see the language reference manual for details), and the standard
+  library is fairly reasonable.
+  There is support for Mercury-level debugging, but there is not yet support
+  for Mercury-level profiling.
+  </p>
+  <p>
+  For a guide to installing and using the C# grade, refer to README.CSharp and
+  the User's Guide. You will need Mercury release 11.01 or later.
+  </p>
  </div>
-<div style="float: left; max-width: 260px; margin-right: 50px;">
-    <h3>Native code</h3>
-	<p>
-	We formerly had a back-end that compiled to assembler, using the GCC
-	back-end. It had not been maintained for many years, so has been removed.
-	</p>
-</div>
-<div style="float: left; max-width: 260px;"> 
-	<h3>Microsoft's .NET</h3>
-	<p>
-	This back-end generates IL, the Intermediate Language
-	used by Microsoft's .NET Common Language Runtime.
-	</p>
-	<p>
-	Click <a href="<?php echo $root?>/about/dotnet.html">here</a> for more
-	</p>
  </div>
-
diff --git a/css/common.css b/css/common.css
index 375d719..02b7317 100644
--- a/css/common.css
+++ b/css/common.css
@@ -211,11 +211,16 @@ font-style: italic;
  font-weight: bold;
  }

-.backendssubsec{
-font-size: 16px;
-font-style: italic;
-margin-top: 30px;
-margin-bottom: 0;
+.backend-container {
+  display: flex;
+  flex-wrap: wrap;
+  column-gap: 5%;
+  justify-content: space-between;
+}
+
+.backend-item {
+  flex: 0 47%;
+  height: 100%;
  }

  pre{