Use copilot markers in PR template (#1784)

* Use copilot markers in PR template

* remove poem

Co-authored-by: Clement Rey <cr.rey.clement@gmail.com>

---------

Co-authored-by: Clement Rey <cr.rey.clement@gmail.com>

Author: emilk

.github/pull_request_template.md

@@ -1,3 +1,13 @@
+### WHAT
+copilot:summary
+​
+
+### WHY
+<!-- author to complete -->
+
+### HOW
+copilot:walkthrough
+
 ### Checklist
 * [ ] I have read and agree to [Contributor Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and the [Code of Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
 * [ ] I've included a screenshot or gif (if applicable)