Rebuild the API docs using api-header-extractor script, and add the
script to the normal build for Core so that it doesn't get out of sync
again.
Hotdoc configuration has not been used for a while; it is currently
broken and we haven't got any docs generated by it anyway, so remove
from repo.
Remove largely duplicate BUILDING.md and old json-schema.md.
Test-bot: skip
Turns out we have a document for the API changes, in addition to the
release notes. That document got overlooked so far. This change now
updates that doc with the API changes that happened since v17.
Build-bot: skip
Test-bot: skip
Follow-up-of: #15471
Follow-up-of: #12769
