Create separate CKEditor branches and versions for 12.0 and 12.1

The Hippo CKEditor fork in Github has a hippo-specific branch per CKEditor release (e.g. hippo/4.7.x for CKEditor release 4.7.x). But there are no separate release branches yet for different minor releases of Hippo. For example, because 12.0 and 12.1 both use the hippo/4.7.x branch, changes on that branch are 'automatically' part of both 12.0 and 12.1. That's undesirable, especially since the CKEditor fork now also contains a 'config' module with Java code that will change in upcomining Hippo releases.

The Hippo CKEditor code base should therefore get separate branches per Hippo maintenance release, with separate versions per branch (just like we have for all other modules). These changes should be documented in the README.md file.