doc: organize chapters into parts, and reduce toc depth

Reorganize the chapters into parts and reduce the TOC depth to make the
TOC useful again. The top-level TOC is very brief, but that is fine
because every part will have its own TOC.

Section titles of languages/frameworks are also simplified to just
the name of the language/framework.

doc/languages-frameworks/emscripten.section.md

@@ -1,4 +1,4 @@
-# User's Guide to Emscripten in Nixpkgs
+# Emscripten
 
 [Emscripten](https://github.com/kripken/emscripten): An LLVM-to-JavaScript Compiler
 

doc/languages-frameworks/haskell.section.md

@@ -3,7 +3,7 @@ title: User's Guide for Haskell in Nixpkgs
 author: Peter Simons
 date: 2015-06-01
 ---
-# User's Guide to the Haskell Infrastructure
+# Haskell
 
 
 ## How to install Haskell packages

doc/languages-frameworks/idris.section.md

@@ -1,4 +1,4 @@
-# Idris packages
+# Idris
 
 ## Installing Idris
 

doc/languages-frameworks/index.xml

@@ -1,7 +1,7 @@
 <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>
+ <title>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>

doc/languages-frameworks/node.section.md

@@ -1,5 +1,5 @@
-Node.js packages
-================
+Node.js
+=======
 The `pkgs/development/node-packages` folder contains a generated collection of
 [NPM packages](https://npmjs.com/) that can be installed with the Nix package
 manager.

doc/languages-frameworks/r.section.md

@@ -1,5 +1,5 @@
-R packages
-==========
+R
+=
 
 ## Installation
 

doc/languages-frameworks/rust.section.md

@@ -4,7 +4,7 @@ author: Matthias Beyer
 date: 2017-03-05
 ---
 
-# User's Guide to the Rust Infrastructure
+# Rust
 
 To install the rust compiler and cargo put
 

doc/languages-frameworks/vim.section.md

@@ -3,7 +3,7 @@ title: User's Guide for Vim in Nixpkgs
 author: Marc Weber
 date: 2016-06-25
 ---
-# User's Guide to Vim Plugins/Addons/Bundles/Scripts in Nixpkgs
+# Vim
 
 Both Neovim and Vim can be configured to include your favorite plugins
 and additional libraries.