Add explicit general committer role

When doing this, do we want to update also this part, to include general committers?

The roles of Project Lead, product manager, framework manager, and release manager have commit access and are collectively referred to as "core committers" (or "branch maintainers" for a specific major Drupal version).

It seems to me like a duplicate of the "Leadership Team: Core Committers" part, but given the roles are explicitly mentioned here, then it is probably not ideal to keep it as is.