Automate the monthly docs release process

Automate the process of releasing the GitLab product docs, a process outlined in: https://docs.gitlab.com/ee/development/documentation/site_architecture/release_process.html.

It might mean:

• Rationalizing the Docker images we maintain: https://gitlab.com/gitlab-org/gitlab-docs/-/tree/master/dockerfiles
• Versioning this project, so that we don't have to worry about breaking changes that often affect stable branches of GitLab that rely on old logic.
• Implementing some form of ChatOps, so that we can run chat commands for common tasks.
• Automate processes that are currently manual (Example: #667 (closed))