Building the nixpkgs manual currently triggers a bunch of deprecation
warnings, because every attribute in `lib` is evaluated to see if it's
an attrset to generate locations for.
Instead, share the lib subsets to include in the documentation
between `lib-function-docs` and `lib-function-locations` so they can
coordinate.
Also generate the list of sections instead of duplicating it in
`library.xml`.
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.
