Update pull request template (#6333)

## Motivation for features / changes
This just looks better

## Technical description of changes
* I updated our PR template to improve the formatting.
* ~I added a new GitHub Actions workflow which runs a custom node
script~
* ~The script attempts to parse the PR description to ensure all
sections specified in the template exist and have content~

## Screenshots of UI changes (or N/A)
N/A

Author: rileyajones

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,9 @@
-* Motivation for features / changes
+## Motivation for features / changes
 
-* Technical description of changes
+## Technical description of changes
 
-* Screenshots of UI changes
+## Screenshots of UI changes (or N/A)
 
-* Detailed steps to verify changes work correctly (as executed by you)
+## Detailed steps to verify changes work correctly (as executed by you)
 
-* Alternate designs / implementations considered
+## Alternate designs / implementations considered (or N/A)