Update pull_request_template.md

Add testing/documentation items

Author: cscheid

.github/pull_request_template.md

@@ -24,4 +24,7 @@ I have (if applicable):
 
 - [ ] filed a [contributor agreement](https://github.com/quarto-dev/quarto-cli/blob/main/CONTRIBUTING.md).
 - [ ] referenced the GitHub issue this PR closes
-- [ ] updated the appropriate changelog
+- [ ] updated the appropriate changelog in the PR
+- [ ] ensured the present test suite passes
+- [ ] added new tests
+- [ ] created a separate documentation PR in [Quarto's website repo](https://github.com/quarto-dev/quarto-web/) and linked it to this PR.