nixpkgs manual: split languages into separate files

There's no change in content except for amending the title of the
section to mention "frameworks", as e.g. I don't consider Qt a language,
and it's likely there will be more of similar cases in future.

To be certain, I checked diff of the generated HTMLs.

doc/languages-frameworks/index.xml

@@ -0,0 +1,42 @@
+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xi="http://www.w3.org/2001/XInclude"
+         xml:id="chap-language-support">
+
+<title>Support for specific programming languages and frameworks</title>
+
+<para>The <link linkend="chap-stdenv">standard build
+environment</link> makes it easy to build typical Autotools-based
+packages with very little code.  Any other kind of package can be
+accomodated by overriding the appropriate phases of
+<literal>stdenv</literal>.  However, there are specialised functions
+in Nixpkgs to easily build packages for other programming languages,
+such as Perl or Haskell.  These are described in this chapter.</para>
+
+
+<xi:include href="perl.xml" />
+<xi:include href="python.xml" />
+<xi:include href="ruby.xml" />
+<xi:include href="go.xml" />
+<xi:include href="java.xml" />
+<xi:include href="lua.xml" />
+<xi:include href="coq.xml" />
+<xi:include href="qt.xml" />
+
+
+<!--
+<section><title>Haskell</title>
+
+<para>TODO</para>
+
+</section>
+
+
+<section><title>TeX / LaTeX</title>
+
+<para>* Special support for building TeX documents</para>
+
+</section>
+-->
+
+
+</chapter>