Match Markdown link reference labels case-insensitively

[afurm]

Markdown link reference labels are matched case-insensitively (both in the original Markdown syntax and in CommonMark), but RDoc::Markdown stored and looked up references by their exact text. So with

See [the Docs][] and [Ruby].

[the docs]: https://docs.ruby-lang.org
[ruby]: https://www.ruby-lang.org

both links were left as literal [the Docs][] and [Ruby] in the output.

The label text is already normalized for whitespace in the Label rule, so I added a small reference_key helper that downcases the label, and used it both where a reference is stored and where a link looks it up. The link text keeps its original case.

I changed markdown.kpeg and regenerated markdown.rb with rake generate; rake verify_generated, rake normal_test and rubocop pass locally.

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Commit: b987fd7