Hide embedded Racc runtime from generated API docs

[OskarEichler]

Summary

Wrap the Racc runtime embedded in the generated RD parsers with :stopdoc: / :startdoc: markers, and teach the generator to reproduce those markers.

This keeps the generated RDoc::RD API while excluding the bundled parser-runtime implementation from RDoc output.

Fixes #1762.

Verification

• generated parser files reproduce cleanly;
• complete suite: 2,529 tests / 6,181 assertions on Ruby 3.2 and 4.0;
• RubyGems integration: 20 tests / 46 assertions;
• full self-documentation generation and focused documentation model;
• 209-file RuboCop, CSS/template lint and syntax checks.

No test files were changed. Parser runtime behavior is unchanged; only generated documentation visibility changes.

[st0012]

We shouldn't update generated docs this way. Can we exclude them in .rdoc_options instead?