Add PR template (#1820)

## Summary

Add a PR template. The idea is to make PR changesets accessible to people who may not be aware of all areas of the product, so in a way it also helps with their understanding. Also applies with any new team members we may onboard.

.github/pull_request_template.md

@@ -0,0 +1,40 @@
+## Summary
+
+<!--
+Describe what changed and why.
+Write for reviewers who may not be familiar with this area of the product.
+-->
+
+### Screenshots or video
+
+<!--
+If this PR includes UI changes, include screenshots or a short video.
+For new features, "Before" can be omitted.
+
+Omit this section if the PR does not contain any UI changes.
+-->
+
+| Before | After |
+| :----- | :---- |
+|        |       |
+
+### How to test locally or on Vercel
+
+<!--
+List clear, reproducible steps to validate this change, either locally or on Vercel.
+Include any setup needed, such as feature flags, env vars, seed data, or migrations.
+-->
+
+1.
+2.
+3.
+
+### References
+
+<!--
+Add any supporting references that help reviewers understand this PR.
+Examples: issue/ticket or related PRs.
+-->
+
+- Linear Issue:
+- Related PRs: