Automate guide versioning steps when creating a new release

[thibaudcolas]

We currently document our step-by-step process to create a new version of the documentation here: https://guide.wagtail.org/en-latest/contributing/#versioning

There are quite a few steps involved, and some take a lot of time: copying all images in particular. It would be nice to get some automation involved so this isn’t as time-consuming.

Link updates from the CMS we could automate:

• Create a redirect view for future releases not set up on the site yet #306

Guide site updates we could automate:

• Bulk-copy images from the "latest" collection to a "vx.y.z" collection
• Bulk-update image references in copied pages from "latest" to their expected version / collection

Note all images for the latest release need updated visuals every 3 months anyway, so it might make sense to revisit this based on exact workflow.