docs: add the Backport page for the /backport comment command - Add the Backport page: installing, the command, the backport pull request, failure cases, reactions and why the merge trigger is pull_request_target - List backport.yml in the Workflows table and link the page from the sidebar - Point the manual cherry-pick in Releasing to the backport command - Name the backport command as a user of the GitHub App - Add /backport and pull_request_target to the Glossary and extend backport and author association Signed-off-by: ernolf <raphael.gradenwitz@googlemail.com>