Add version selector

[p8]

To make it easier to switch to documentation of other versions, add a select box to select other versions.

For example, the versions can be defined in .rdoc_options:

versions:
  latest: https://docs.ruby-lang.org/en/
  master: https://docs.ruby-lang.org/en/master/
  3.4: https://docs.ruby-lang.org/en/3.4/

This would allow https://docs.ruby-lang.org/en/ to just show RDoc documentation for the last released version instead of a list of documentation version.

[github-actions[bot]]

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[st0012]

Thanks for the PR! Can this feature benefit https://api.rubyonrails.org/ too?