cop.documentation_url is nil for extension cops

[Earlopain]

Is your feature request related to a problem? Please describe.

This one here:

rubocop/lib/rubocop/cop/base.rb

Lines 63 to 69 in 7f9595a

	# Cops (other than builtin) are encouraged to implement this
	# @return [String, nil]
	#
	# @api public
	def self.documentation_url
	Documentation.url_for(self) if builtin?
	end

It only returns something for cops in this repository. The Ruby LSP makes use of this method to link to a cops documentation in code lenses:

The blue link takes me to https://docs.rubocop.org/rubocop/cops_style.html#stylestringliterals

Here is how it looks for rubocop-rails (and basically all other extension) cops:

The method has the following comment:

Cops (other than builtin) are encouraged to implement this

This hasn't happenend yet. I'd like to do so, however I don't really know how I would do that since the Documentation class is private.

Describe the solution you'd like

rubocop --show-docs-url Rails/Exit prints the correct url, so I wanted to just copy how it does that over. However I don't know how the get to the config/config store in the context of that method without requiring users to pass it in themselves.