chore: reformat pull request template (#28923)

### Reason for this change

I noticed that some PRs only link the issue or have really long paragraph descriptions which can be hard to follow and understand the why/what regarding the changes.

### Description of changes

Reformatting the pull request template with section headings. This enforces PR descriptions to follow a uniform format and include the information we want. Should make it easier/more efficient to review PRs. 

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,16 +1,22 @@
-> REPLACE THIS TEXT BLOCK
->
-> Describe the reason for this change, what the solution is, and any
-> important design decisions you made. 
->
-> Remember to follow the [CONTRIBUTING GUIDE] and [DESIGN GUIDELINES] for any
-> code you submit.
->
-> [CONTRIBUTING GUIDE]: https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md
-> [DESIGN GUIDELINES]: https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md
+### Issue # (if applicable)
 
 Closes #<issue number here>.
 
+### Reason for this change
+
+<!--What is the bug or use case behind this change?-->
+
+### Description of changes
+
+<!--What code changes did you make? Have you made any important design decisions?-->
+
+### Description of how you validated changes
+
+<!--Have you added any unit tests and/or integration tests?-->
+
+### Checklist
+- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)
+
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 *By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*