Add new group change template file

This template makes it easier for us to track the TW tasks required when a group change (rename, group split etc) takes place. These do not usually result in a TW reassignment, but they do affect docs metadata, dashboards, and Workday.

We'll use this template for group changes triggered by PMs or EMs, for example: Update data/stages.yml to split Package into tw... (gitlab-com/www-gitlab-com!112958 - merged)

If a TW stable counterpart change is triggered by the TW team, for example as part of a team rebalance, we'll continue to use the transition template.