[llvm] [Docs] Update documentation for the new GitHub workflow (PR #65162)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

Thanks @tru, it's fine to follow-up on my comments and other after the week-end: having Phabricator still documented as "the way to go" is worse than having an imperfect GitHub documentation in the meantime.

https://github.com/llvm/llvm-project/pull/65162