Commit generated files

[st0012]

Closes #1107 and will likely prevent issues like https://bugs.ruby-lang.org/issues/21541 in the future

With ignoring generated files in the source control, we always need one extra step to make the gem usable, which limits how the gem can be distributed (#1107) and make post-distribution issues easier to happen (which are in general harder to fix too).

If gems like prism and rbs and debug can avoid this issue by committing generated files and verifying them on CI, I think we should do that too.

[matzbot]

🚀 Preview deployment available at: https://6c6484b3.rdoc-6cd.pages.dev (commit: df6379b)

[kou]

+1

[tompng]

I agree for adding generated files 👍

CI seems to pass even if generated files are out of date.
Diff to markdown.kpeg: https://github.com/tompng/rdoc/actions/runs/16994310600 All tests passes
Diff to block_parser.ry: https://github.com/tompng/rdoc/actions/runs/16995015009 macos-latest always fails, windows-latest always passes.

Reproduce locally:

1. Add a diff to markdown.kpeg and commit. verify_generated correctly fails.
2. Switch to another branch and go back again. verify_generated wrongly passes.
3. touch markdown.kpeg and try rake verify_generated again. It correctly fails.