[clang] [CIR] Refresh the ClangIR landing docs page (PR #232501)

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

> Also, to remove the 'warning', we have to wait until #221291 merges.

Good point, added the warning back (without the incubator part) and added extra info note in the "Using ClangIR" section with instructions to enable it in CMAKE. Should be easy to remove once it lands.

https://github.com/llvm/llvm-project/pull/232501