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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> 1. Clang's documentation should consistently be in .rst files; there have been a few things moved to `.md` but those changes have been questioned enough that we should not use it as precedent for adding more .md files.

This comment surprised me. My understanding is that LLVM documentation is moving towards markdown. Why don't we want to do this with Clang?

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