add helpdb and associated Lua API

[myk002]

#2224 Merge the helpdb logic into develop

of course it's not going to serve the highest quality data without also merging the doc updates from the docs branch, but it's a chunk that can be reviewed and merged separately, so we should do so. Moreover, it does work perfectly fine with existing in-cpp plugin docs and in-lua/rb script docs since it supports the legacy formats.