[FEAT] new env var and mount point: PLUGINS_PATH

Review the repository documentation and container configuration, then compare them with the referenced Manyfold example Dockerfile. Document how Linuxserver users should configure PLUGINS_PATH and mount persistent storage, including the /config/plugins example. Done means the recommended setup is clear without changing application behavior.

Is this a new feature request?

• I have searched the existing issues

Wanted change

Linuxserver images need a new volume mount for persistent plugin storage - most likely this is a documentation change.

Reason for change

I've added a new plugin system to Manyfold (coming in v0.146.0). In order to last between container updates, the plugins need to be stored somewhere persistent. I've added a PLUGINS_PATH environment variable which defaults to the app's plugins directory, but it needs persistent storage mounted at that location, which I've mentioned in our example dockerfile (see https://github.com/manyfold3d/manyfold/pull/6569).

Proposed code change

The app will work without persistent storage mounted, and should show a warning on the plugin screen if it's not writable, so nothing should break; however, I imagine you'll want to add how users should set this up in a linuxserver way in your docs. Normally I'd do a PR myself, but I don't want to make assumptions on what you want your users to do.

As an example, in our single-container "solo" image, I've preconfigured the path to /config/plugins, as there's already an SQLite database at /config and users will already have a volume mounted there that we can use.