Move config from Rails to SDoc

[p8]

Currently generating the API docs is integrated in the Rails project:
https://github.com/rails/rails/blob/main/railties/lib/rails/api/task.rb
This requires the sdoc gem to work with the supported Ruby version of the Rails project and its Gemfile.
If docs generation can be run seperately we can generate the docs for any version of Rails.

[zzak]

Yeah, this is a known issue but I'm not sure where to put it yet.

[p8]

I think we could have some config file in sdoc where we define the settings for each version of Rails.

[p8]

RDoc supports a .rdoc_options file
We could move a lot of options there.