Set up parallel deployments for released versions

Now that parallel deployments is in beta, we can use this feature to replace some things we've done previously with Docker files: running the online docs for recent versions on the main site (e.g, https://docs.gitlab.com/17.6/), and also online archives at https://archives.docs.gitlab.com.

How this is different from our Nanoc deploys:

• No need for latest.Dockerfile
• To create a release, all we need is the versioned branch to be created/pushed, and then an update to versions.json for the Versions dropdown menu
• The space constraints we had around running so many copies of the site from the same project/domain name are no longer an issue

We will still need to create the version-specific Dockerfiles to support customers who self-host the docs using these packages, but it will simplify how we run online older versions, and how we cut a release.

We have a few things to figure out:

• There will also be something we need to do with Hugo config to make sure the paths still work (similar to how we build Hugo when creating the Docker archives). How could we have Hugo build with different config on the versioned archive branches?
• How does this affect the versions dropdown? We no longer need versions to move from docs.gitlab.com over to archives.docs.gitlab.com.
• How does this simplify the process of backporting docs changes?