Improve the pull request template (#1625)

Author: mdlinville

.github/pull_request_template.md

@@ -1,10 +1,23 @@
-Description
------------
-<!-- Description of the change -->
+## Description
 
+<!-- Uncomment and add a description of the change -->
 
-Related issues
------------
-- Fixes DOCS-12345 <!-- JIRA issue, or remove this line -->
-- Fixes #12345 <!-- GitHub issue, or remove this line -->
 
+<!-- Optionally, uncomment the heading and add details about how you tested the change and how reviewers should test it. For example:
+## Testing
+- [ ] Local build succeeds without errors or broken internal links (`hugo server`)
+- [ ] PR tests succeed
+- [ ] The Lychee Github action run against the PR branch reports no broken links
+
+Replace the `[ ]` with `[x]` to check off the item instead of leaving it unchecked.
+
+Otherwise, delete this entire section from opening to closing comment.
+-->
+
+<!-- Optionally, uncomment the heading and add one or more lines like these. Otherwise, delete this entire section from opening to closing comment.
+
+## Related issues
+
+- Fixes DOCS-12345
+- Fixes #12345
+-->