Generate documentation for prepare/feature plugins

[happz]

Pull Request Checklist

A preview: https://tmt--3438.org.readthedocs.build/en/3438/plugins/prepare-feature.html

• implement the feature

[thrix]

@happz I am not super satisfied how it looks now, I think it would make more sense to have the plugins listed under Prepare plugins, under the Feature prepare plugin

Prepare -> feature -> Plugins (next to configuaration).

While I understand prepare/feature are also plugins, but now with two sections, I find it confusing.

[happz]

@happz I am not super satisfied how it looks now, I think it would make more sense to have the plugins listed under Prepare plugins, under the Feature prepare plugin

Prepare -> feature -> Plugins (next to configuaration).

While I understand prepare/feature are also plugins, but now with two sections, I find it confusing.

I'm open to suggestions. @psss care to chime in?