Add performance triage policy template

What does this MR do and why?

Describe in detail what your merge request does and why.

Adds a monthly performance issues triage to mirror the bug triage introduced in !1015 (merged). This is limited to the Plan and Create stages currently.

This is the final step of #701 (closed)

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review