[llvm] [CI] Add Github actions job to build LLVM documentation (PR #69269)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

I've validated that this works locally with `act` and am interested in reviewer feedback now. Once this lands I'll work on getting a patch up for building the clang docs.

https://github.com/llvm/llvm-project/pull/69269