Migrate documentation to rdoc

[ydah]

Fixes: #5576

This PR completes the migration of the GraphQL Ruby documentation site from YARD/Jekyll to RDoc/Aliki.

It adds the RDoc build and CI checks, converts guide metadata and YARD comments, moves API-specific guide content into source comments, preserves legacy documentation URLs, and removes the legacy YARD pipeline. The generated API documentation is checked against the pre-migration public API inventory to detect unexpected coverage changes.

The generated site also includes GraphQL syntax highlighting, local-file-friendly image links, correct README rendering, nested guide navigation with page titles, and complete development guide table-of-contents links. No GraphQL runtime behavior is changed.

Validated with bundle exec rake docs:check, the documentation generator specs, the link checker, the RDoc reference checker, and the GraphQL highlighter tests.

[rmosolgo]

Hey, thanks so much for taking a crack at this! I just pulled the branch and built the docs locally. In general I think this is great, but I'd like to get a few more things before replacing the YARD docs with this:

• convert @api private to :nodoc:; right now they're rendered like this:

  

• Type deifnitions, somehow Prism has them in its RDoc: https://docs.ruby-lang.org/en/master/Prism/CallNode.html I would love to convert our YARD to types like that

I'm going to continue reviewing but I thought I'd share those ideas now to see what you think.