chore: reformat pull request template

[gracelu0]

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

[GavinZZ]

nit: I really liked SAM-Transform repo's PR template where we issues as a section and users can always add more detail to the issue section to explain the issue.

### Issue #, if available
Closes #<issue number here>.

Maybe make the checklist its own section and we can append to it in the future

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and the [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

[gracelu0]

nit: I really liked SAM-Transform repo's PR template where we issues as a section and users can always add more detail to the issue section to explain the issue.

### Issue #, if available
Closes #<issue number here>.

Maybe make the checklist its own section and we can append to it in the future

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and the [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

I think having the issue at the top is a good idea. I can add a Checklist heading as well to keep it organized. I believe the entire template used to be a long checklist but it got removed since people weren't really using it - something to keep in mind before we decide to add more checklist items.

[GavinZZ]

I believe the entire template used to be a long checklist but it got removed since people weren't really using it - something to keep in mind before we decide to add more checklist items.

That's a good point actually, I tend to ignore all checklist most of the time as well. Maybe we could have it instead as a USEFUL LINKS section if we find it not useful in the future and just put a couple of useful links CDK has.

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[GavinZZ]

@mergify update

[mergify]

update

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[paulhcsun]

@Mergifyio update

[mergify]

update

✅ Branch has been successfully updated

[moelasmar]

do you think we should add a section for the Breaking changes as mentioned here?

[aws-cdk-automation]

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 69d2b7f
• Result: SUCCEEDED
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[gracelu0]

do you think we should add a section for the Breaking changes as mentioned here?

I think that's a good idea since there's a specific format required, I can add it

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).