[llvm] build-docs: Add option to disable doxygen/sphinx docs (PR #66928)

[Tobias Hieta via llvm-commits]

tru wrote:

> > This PR just adds the flag to disable doxygen builds, I will then later
> > make a PR that changes the actions to first build the normal docs and
> > another job to build the doxygen docs.
> 
> Maybe the doxygen build should be a nightly job?

Yes maybe. But I want to make sure that we build doxygen docs for the release tag. I just need to separate it out, waiting for the job to finish (and sometimes fail!) makes releasing a new version very frustrating.

https://github.com/llvm/llvm-project/pull/66928