Update docs release steps (!3308) · Merge requests · GitLab.org / GitLab Docs

In the latest docs release, we ran into some issues that we could avoid if the steps were clearer. This MR aims to that 🙂

Notably:

Clean up the steps of running the release:single task. You no longer need to manually push to create the branch, this happens automatically by the task.
Re-organize the steps so that we now run a pipeline for the stable docs branch if it has previously failed. We essentially ensure that the Docker image is uploaded to the registry before creating the release MR.
Merge the troubleshooting section of failures of the image:docs-single job.
Add recent example of hunting down a failure due to a navbar item broken link.

Update docs release steps