Switch to using gi-docgen for docs (batch 2)
Another load of commits picked off @matthiasc’s original migrate-to-gi-docgen
branch, reviewed, rebased and tidied up. Follows on from !3591 (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