Check if `doc/plugins.md` is still valid

`doc/plugins.md` hasn't been changed for a long time, we should check if it is still valid or if it should be pulled into the main README or even just be removed.

Start with doc/plugins.md and the main README, then compare the documented plugin information with the current repository state. Determine whether the content is still valid and whether it belongs in the README; done means the documentation has been updated, moved, or removed with a clear decision.

Written by the indexing model from the issue text.