Since Rakudo is a compiler, you have to distinguish between documentation for the compiler itself, and the language it implements.
The main documentation for the compiler itself is its manual page.
Documentation for the Raku language can be found on the Raku documentation website, which includes tutorials for programmers who know other languages. There also exist an introductory tutorial and many other learning resources.
Our IRC chat channel is an excellent way to get help from the members of the community, if you're having trouble with anything.