Define high-level process for cross-Sec initiatives

Why is this change being made?

While not overly common, there are scenarios where the Sec section needs to plan for and implement initiatives that cut across the entire section. This MR defines a lightweight process to handle these scenarios.

Issue lists in scope for changes documented in this MR

• https://gitlab.com/groups/gitlab-org/-/issues/?sort=created_asc&state=opened&label_name%5B%5D=section%3A%3Asec&label_name%5B%5D=group%3A%3Anot_owned
• https://gitlab.com/groups/gitlab-org/-/issues/?sort=created_asc&state=opened&label_name%5B%5D=section%3A%3Asec&label_name%5B%5D=all%20Sec%20groups

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