This PR defining the coordination process more concretely. Here are the changes:

• Changes the process to automatically assign a Coordinator after a period of idleness. The exact selection process isn't defined, at this stage I am looking for reviews on whether this change to the process is acceptable to PSRT members.
• The coordinator role per-ticket is explicit within GHSA using the "Credit: Coordinator" feature.
• Separates the Triage, Remediation, Advisory sub-processes more clearly. This intended to be helpful for non-PSRT members that are involved in the remediation process to know what to expect.
• Documents how the PSRT bot operates so coordinators know what to expect and how to take advantage of the automation.

cc @python/psrt