Document upgrade process for Wagtail

[xmedr]

Documentation request

We're upgrading the CPS SSCE dashboard from Wagtail 2.15 to 6.1 and the official documentation recommends making changes one feature release at a time (4.0 -> 4.1 -> 4.2 etc.). Many of our wagtail projects are set up similarly using older versions, so let's aggregate all the changes that get made to CPS once that's done. That way we can make all those changes in one go per project, as opposed to going through each feature release for all of them.

[hancush]

I'm creating a fresh Wagtail project for the NOAH map. I can use those takeaways to update the cookie cutter, too.