workflows/release-task: Use less privileged token for uploading release notes (#180299)
We were using one token for both pushing to the llvmbot fork and for
creating a pull request against the www-releases repository, since the
fork and the repository have different owners, we were using a classic
access token which has very coarse-grained permissions. By using two
separate tokens, we limit the permissions to just what we need to do the
task.
Also, at the same time run the release-documentation job in an
environment, so we can limit the accessiblity to these secrets.