talkatu/talkatu

Migrate to gi-docgen and remove the doc comments that were gtk-doc specific

Testing Done:
Build the docs locally and via the convey plan.

Reviewed at https://reviews.imfreedom.org/r/1056/
Talkatu is versioned in [mercurial](https://www.mercurial-scm.org/) and is
available on [keep.imfreedom.org](https://keep.imfreedom.org/talkatu/talkatu).
All contributions are provided via review requests at [reviews.imfreedom.org](https://reviews.imfreedom.org/).