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
This change updates the documentation files that get generated out of
the the content in the source files. These files showed up as changed
when building.
Build-bot: skip
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
PR #12769 removed `km_core_keyboard_load` that allowed loading a keyboard
from a path. Instead we now pass a BLOB to core to load a keyboard.
With the function gone there is no longer a need to have a field with the
path.
Follow-up-of: #12497
Related-to: #12769
Test-bot: skip
