chore(core): refresh documentation and add build tool

Refreshes all of the public API header documentation so we have a single
source for the documentation -- the keyman_core_api.h header. A lot of
reformatting but very little change in terms of content.

This is converted into Markdown by api-header-extractor, and the results
can be copied to help.keyman.com.

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.