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

This patch adds in support for building the LLVM documentation through a
Github actions job. This enables catching documentation build failures
earlier and also more easily as the job failure will show up directly on
pull requests. The job currently only builds the documentation for LLVM,
but the plan is to extend it to also build the documentation for other
subprojects when appropriate (i.e., the docs files have changed),
starting with clang.