Generate all Ruby versions' documentation with the latest (released) RDoc

[st0012]

I recently made quite a few theme improvements to RDoc, along with community contributed changes like SEO tags...etc., which are all applied to the master documentation (https://docs.ruby-lang.org/en/master/index.html).
But since most of users will still be using Ruby 3.3 and prior versions for years to come, I think it'd be better if we can apply the same changes to their documentation too. I'd appreciate some pointers or even collaborations to make that happen. Thank you 🙂