[llvm] [Github] Add steps to build clang docs to CI (PR #69550)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

I've set everything up in the same job for now mostly because it was simple. We can split it into multiple jobs if necessary, but I don't think many patches touch the docs of multiple subprojects, and those that do still won't run into any problems with the job taking too long. From my testing on my fork, building both the LLVM and clang docs takes under 10 minutes, even with clang having to build `clang-tblgen`.

https://github.com/llvm/llvm-project/pull/69550