doc: Add automatic generation of library function documentation

Modifies the build process of the manual to invoke nixdoc
automatically to generate XML files with function documentation.

Currently documentation is present for five of the files in `lib/`.

To add another file to the generated docs, both
`doc/functions/library.xml` and `doc/lib-function-docs.nix` must be
updated.

doc/default.nix

@@ -2,8 +2,8 @@
 let
   lib = pkgs.lib;
   locationsXml = import ./lib-function-locations.nix { inherit pkgs nixpkgs; };
-in
-pkgs.stdenv.mkDerivation {
+  functionDocs = import ./lib-function-docs.nix { inherit locationsXml pkgs; };
+in pkgs.stdenv.mkDerivation {
   name = "nixpkgs-manual";
 
   buildInputs = with pkgs; [ pandoc libxml2 libxslt zip jing  xmlformat ];
@@ -32,6 +32,7 @@ pkgs.stdenv.mkDerivation {
   postPatch = ''
     rm -rf ./functions/library/locations.xml
     ln -s ${locationsXml} ./functions/library/locations.xml
+    ln -s ${functionDocs} ./functions/library/generated
     echo ${lib.version} > .version
   '';