Use the document link request to link to Rails documentation

[egiurleo]

We implemented the DocumentLink request in #58 and used it to go to method definition from gem RBIs.

As a next step, let's use this request to link to the correct Rails documentation for methods that come from the rails gem.

When a user hovers over a method that's defined in rails, there should be a link that says "go to documentation." We can use information about the method, such as the class that defines it and its ancestor chain, as well as information about the rails gem (version number) to link to the correct documentation page for that method.