Add PR template

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.github/PULL_REQUEST_TEMPLATE.md

+### Description
+
+<!-- Describe your changes in detail -->
+
+### Motivation & Context
+
+<!-- Why is this change required? What problem does it solve? -->
+
+### Type of changes
+
+<!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply. -->
+
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Refactoring (non-breaking change)
+- [ ] Breaking change (fix or feature that would change existing functionality)
+
+### Checklist
+
+<!-- Go over all the following points, and put an `x` in all the boxes that apply. -->
+<!-- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
+
+- [ ] I have read the [contributing guidelines](https://github.com/twbs/bootstrap/blob/main/.github/CONTRIBUTING.md)
+- [ ] My code follows the code style of the project _(using `npm run lint`)_
+- [ ] My change introduces changes to the documentation
+- [ ] I have updated the documentation accordingly
+- [ ] I have added tests to cover my changes
+- [ ] All new and existing tests passed
+
+#### Live previews
+
+<!-- Please add direct links where your modifications can be seen in the documentation -->
+
+* https://deploy-preview-{your pr number}--twbs-bootstrap.netlify.app/
+
+### Related issues
+
+<!-- Please link any related issues here. -->