Remove YARD-to-RDoc migration tooling

[rmosolgo]

Follow-up to #5689. Now that the docs migration is done, this removes code that only existed to drive or verify it (~1,700 lines).

Removed

• tool/docs/migrate_guides.rb and its spec: converted Jekyll/Liquid guides to RDoc markdown. Every guide is converted, and tool/docs/check.rb already rejects new Liquid tags.
• tool/docs/guide_audit.rb, docs/guide_classification.yml and the spec: verified that API guides moved into source comments.
• docs/yard_api_baseline.yml and the --baseline comparison in tool/docs/compatibility.rb. The checks that the RDoc search index has no duplicates and every entry links to a real page and anchor are unchanged.
• # migrated from … markers in lib/graphql/{query,schema,schema/argument,schema/field}.rb, which only guide_audit.rb read.
• References to the old guides/yardoc/ / Jekyll directories in .gitignore, tool/docs/build.rb and generator_spec.rb.
• The migrate_guides / guide_audit / --baseline steps in rake docs:check.

Kept on purpose

• tool/docs/type_signatures.rb: its --check still enforces :call-seq: on new methods.
• tool/docs/legacy.rb (LegacyAnchors): keeps old YARD-style method anchors working for inbound links.
• tool/docs/redirects.rb / docs/redirects.yml: keep old guide URLs working.

Verification

Run locally:

• docs specs
• rake docs:check
• rake docs:build_twice
• tool/docs/gem_install_check.rb
• the versioned-API steps for master (build_version, version_check, compatibility --strict, rdoc_ref_checker)
• rubocop on the changed files

🤖 Generated with Claude Code

[rmosolgo]

cc @ydah -- I believe we're all done with this code now that YARD is gone. Please let me know if I removed something that we still benefit from!

I kept tool/docs/compatibility.rb because we generate some custom pages that might produce unusual output with RDoc's search index.