Add section about official Ruby Language Documentation

[lucianghinda]

Context

1. In my experience, not many developers use the docs.ruby-lang.org documentation as their main point of reference about the language, and people occasionally ask what the official documentation is.

For this I think it is good to move the link to the official documentation at the top of the page.

2. I also discovered that there are people who, when reading API documentation, automatically think about web API and do not consider that API is an Application Programming Interface. When we usethe term here API documentation, we refer to the documentation about how to use the language.

For this, I think it is good to use a wording that says "the Ruby language documentation" instead of "the Ruby API documentation"

Changes

• Introduce new section highlighting docs.ruby-lang.org as the authoritative source for Ruby language documentation

• Clarify that this should be the first reference for learning Ruby or when having the need to understand how Ruby works

• Explain that other resources are complementary or present documentation in different styles

• Try to help quickly identify the official Ruby language documentation for the reader by removing an acronym and replacing it with an explicit wording

PS: I am not an English speaker, so if anyone can help with better phrasing, please do so.

[st0012]

Both link 13 and 15 are pointing to outdated source on ruby-doc.org: https://ruby-doc.org/3.2.2/, which has the same content as docs.ruby-lang.org. Since they don't serve additional information nor have better UX (IMO), I think we can remove them to shorten the list.

Additionally, link 17 (https://rubydocs.org/) is blank and link 18 (https://apidock.com/) only has documentation up to 2.6. So they should be removed as well.

(I'm happy to open a separate PR to remove them if it's out of the scope of this PR)

[st0012]

I opened #3355 to remove them.