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

[via llvm-commits]

cor3ntin 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?

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