Add hints to pull request template & ask for screenshots

Updated the pull request template to improve clarity and structure.

Author: clairep94

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,13 @@
-Fixes #issue-number
+### Fixes issue-number:
+<!-- Please add the issue this relates to -->
 
-Changes:
+### Demo:
+<!-- Please add a screenshot for UI related changes -->
 
-I have verified that this pull request:
+### Changes:
+<!-- Summarise your changes -->
+
+### I have verified that this pull request:
 
 * [ ] has no linting errors (`npm run lint`)
 * [ ] has no test errors (`npm run test`)