chore: update release note generation formatting (mongodb#3738)

Author: dariakp

.github/pull_request_template.md

@@ -16,11 +16,11 @@ If you haven't already, it would greatly help the team review this work in a tim
 You can do that here: https://jira.mongodb.org/projects/NODE
 -->
 
-#### Release Highlight
+### Release Highlight
 
 <!-- RELEASE_HIGHLIGHT_START -->
 
-## Fill in title or leave empty for no highlight
+### Fill in title or leave empty for no highlight
 
 <!-- RELEASE_HIGHLIGHT_END -->
 

.github/scripts/highlights.mjs

@@ -52,7 +52,7 @@ async function getPullRequestContent(pull_number) {
     return '';
   }
 
-  const start = body.indexOf('## ', body.indexOf(startIndicator));
+  const start = body.indexOf('### ', body.indexOf(startIndicator));
   const end = body.indexOf(endIndicator);
   const highlightSection = body.slice(start, end).trim();
 
@@ -67,6 +67,9 @@ async function pullRequestHighlights(prs) {
     const content = await getPullRequestContent(pr);
     highlights.push(content);
   }
+  if (!highlights.length) return '';
+
+  highlights.unshift('## Release Notes\n\n');
   return highlights.join('');
 }