docs: add pull request template

.github/pull_request_template.md

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+<!-- Provide a general summary of your changes in the title above. -->
+
+# Description
+<!--
+What do you want to achieve with this PR? Why did you write this code? What problem does this PR solve?
+Describe your changes in detail and, if relevant, explain which choices you have made and why.
+When making changes to the UI, make sure to include comparison screenshots!
+-->
+
+## Related issues/external references
+<!--
+Format issues on GitHub as `#XXX`. Tickets from support.gewis.nl can also be auto-linked by using
+`ABC-YYMM-XXX`.
+-->
+
+## Types of changes
+<!-- What types of changes does your code introduce? Remove all the items that do not apply: -->
+- Bug fix _(non-breaking change which fixes an issue)_
+- New feature _(non-breaking change which adds functionality)_
+- Breaking change _(fix or feature that would cause existing functionality to change)_
+- Documentation improvement
+- Style _(Change that do not affect the functionality of the code)_
+- CI/CD _(Changes to the CI/CD configuration)_
+- Refactoring _(Changes that neither fixes a bug nor adds a feature)_