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`.
If all the docs are auto-generated, it should be easier to convert
them to Commonmark.
Co-Authored-By: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-Authored-By: Silvan Mosberger <contact@infinisil.com>
