[llvm] workflows: Refactor release-tasks.yml (PR #69523)

[Tobias Hieta via llvm-commits]

tru wrote:

Thanks for doing this. I had some local changes very similar to this. One thing I think we should do is to spilt the build docs step as well. I have added flags to build just sphinx and only doxygen already. The idea is that oxygen takes almost an hour to run while sphinx is done in 10 minutes. And when doing a release the critical wait is for the sphinx step since that contains the release notes. I think it would be good to separate these steps and make it easier to just rerun one of them if needed. 

https://github.com/llvm/llvm-project/pull/69523