Prepare 6.x branch and document procedures on how to contribute

Problem/Motivation

Let's create the 6.x branch which will eventually be merged into Drupal core, planned for the 11.3 release later in 2025. The 3.x release should be marked "unsupported" at that time as we can't support 4 branches in parallel.

At the same time, write up the instructions on how the active branches 4, 5, and 6 will be maintained for that period of time. In particular, this needs to address the question, how issues (already open or newly created) will be handled. Are they being worked on in 5.x and then merged upstream into 6.x?