Standard for manual config

Use index.adoc in the ROOT folder.

Yes

Drop the start-page key.

No. This is possible, but I have been using the manuals/ prefix EDIT: yes.

Simplify the folder structure.

Yes, by dropping the directory below manuals. For example the manual/polar_pi is gone, it's just manual.

If a plugin is still using a fork, make the PR. If/when accepted change sources.state to reflect where the plugin manual is now coming from. Working towards no opencpn-manuals forks?

Yes, eventually there should be no opencpn-manuals forks.

Is there a reference plugin tha

Any of Rick's which have accepted my PR, for example polar_pi