docs: Split dependencies from related libraries

docs: Split dependencies from related libraries

We currently list everything as a dependencies, regardless of whether
it actually is; this is a source of confusion for users that read the
GTK documentation.

Gi-docgen has a new "related" key in the project configuration which
allows us to list libraries that are merely related to the namespace
we are documenting; the "dependencies" key is used to document the
actual namespace dependencies, now.