[clang] [clang][CIR][doc] Add auto-generated ClangIR documentation (PR #190354)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM with a request for a follow-up, presuming the CIR folks are happy with the state of things.

https://github.com/llvm/llvm-project/pull/190354