The only official Ruby documentation exists at https://docs.ruby-lang.org/en/ and is generated via the Ruby source code itself.
The excellent Colby Swandale has been working on a new open source alternative for Ruby documentation considering the design shortcomings of the default generated API documentation with https://rubyapi.org which I highly recommend.
Sadly there are a lot of third-party operated documentation sites that exist likely because of how hard-to-discover the official API documentation site is. It's something the community would like to address, but given the amount of link equity a lot of these older and often unmaintained doc sites have acquired it's fairly tricky.
The only official Ruby documentation exists at https://docs.ruby-lang.org/en/ and is generated via the Ruby source code itself.
The excellent Colby Swandale has been working on a new open source alternative for Ruby documentation considering the design shortcomings of the default generated API documentation with https://rubyapi.org which I highly recommend.
If you're coming at Ruby from another language, the official Ruby site offers https://www.ruby-lang.org/en/documentation/ruby-from-other-l... and other documentation aside from API concerns here: https://www.ruby-lang.org/en/documentation/
Hope that helps.