Create offboarding template for staff members

Added a template for offboarding staff members, including steps for removal from the team and email decommissioning.

Author: rrrutledge

.github/ISSUE_TEMPLATE/offboard-staff.md

@@ -0,0 +1,10 @@
+---
+name: Offboard a staff member
+about: When a staff member leaves the InnerSource Commons
+title: Offboard Staff Member
+assignees: rrrutledge
+
+---
+
+- [ ] Remove from [staff team](https://github.com/orgs/InnerSourceCommons/teams/staff).
+- [ ] Decommission `@innersourcecommons.org` email.