docs(core): rebuild Core docs from .h and remove broken hotdoc config 🔱

[mcdurdin]

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

[keymanapp-test-bot]

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