Implementing section in Governance Model: Core team governance

The Core Team governance hasn't been particularly clear in the past, especially in relation to:

• Adding and removing core team members

• Defining what constitutes a core team member and how that relates to other roles such as maintainers, admins, contributors, and the security team

• The expectations for people who hold privileged roles within the Product Team, and from contributors

The Core Team has proposed, following a debate here, some changes to the governance model which will help to clarify this.

These changes are available for review in a PR here: https://github.com/mautic/mautic-community-handbook/pull/303.

Following the acceptance of this proposal the Core Team and Security Team will review its members and remove those who are no longer deemed to be actively participating.