[llvm][docs] Release process improvements (#175759)
Some improvements from working through this getting LLVM 22 off the
ground. Most of the changes are pretty straightforward, but one worth
mentioning is the change to push the trunk version bump and tag before
creating the release branch.
This avoids a scenario that was hit for 22 where the release/22.x branch
was pushed first and more commits landed in trunk before the 23.0.0git
version bump was rebased and committed. The commits landed on trunk
during this brief window had the wrong version. This was resolved by
cherry-picking the commits to the release branch.
It's not entirely clear how big of an issue this really is, but if we
can avoid it and make the process less stressful in the process I think
it makes sense to do so.
