Switch to using gi-docgen for docs (batch 4)
Another load of commits picked off @matthiasc’s original migrate-to-gi-docgen
branch, reviewed, rebased and tidied up. Follows on from !3591 (merged), !3634 (merged) and !3645 (merged).
As with the previous MR, I’m going to merge this without additional review because:
- This is review of Matthias’ changes
- Reviewing this stuff is so tedious that it’s really easy to miss mistakes when reviewing; the time is better spent looking at the rendered HTML for problems