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.

Covers assert functions and about half of the attrsets functions.

Some internal consistency around IDs could be improved.