Move all Keyman Core header API documentation to javadoc-style so that
we can get inline documentation and simplify the readability of the
header files.
Update api-header-extractor to support minimal javadoc formatting of the
comments.
Test-bot: skip
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
This documents the state of `km_core_actions.emit_keystroke` for
different keys pressed. Also some cleanup in other docs.
This documents the state after merging #15609 (for ldml keyboards) and
NN (for kmn keyboards).
Follows: #15609
Build-bot: skip
Test-bot: skip
