docs(core): reorganize and automate doc generation

[mcdurdin]

Will only do the main keyman_core_api.h for v17 release, in interests of time. The other headers are primarily internal only and have many deprecated functions also.

Will also leave the full automation of the documentation generation to a future PR. The script will be there but it won't run during build for now.

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(The files json-schema.md and building.md are local to this repo. The remainder are generated from keyman repo, keyman_core_api.h by api-header-extractor, and manually copied in from core/build/doc.)

In the future, we need to move json-schema.md and building.md into the keyman repo as well, and generate the whole folder.

Also in the future we need to eliminate refs to Keyman Core Web and move this entire structure up a level, as it's just "Keyman Core" now, not "Keyman Core Desktop".

Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/issues/10580#issuecomment-2008700376