chore: add pull request template

Adds pull request template to automatically fill out the PR body.

Related to https://linear.app/prefect/issue/PLA-1114/implement-pr-hygiene-improvements

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+### Summary
+
+<!-- Add a brief description of your change here -->
+
+### Requirements
+
+- [ ] [Contributing guide](https://github.com/PrefectHQ/prefect-helm/?tab=readme-ov-file#contributing) has been read
+- [ ] Added/modified configuration includes a descriptive comment for documentation generation
+- [ ] Unit tests are added/updated
+- [ ] Deprecation/removal checks are added in `templates/NOTES.txt`