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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Thanks for the review! I'm going to merge this in a bit and test functionality with a llvm docs patch and start working on support for building the clang documentation tonight.

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