Add a pull request template as a checklist before submitting a PR

.github/PULL_REQUEST_TEMPLATE.md

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+- [ ] I signed and returned the [Plone Contributor Agreement](https://plone.org/foundation/contributors-agreement), and received and accepted an invitation to join a team in the Plone GitHub organization.
+- [ ] I verified there aren't other open [pull requests](https://github.com/plone/volto/pulls) for the same change.
+- [ ] I followed the guidelines in [Contributing to Volto](https://6.docs.plone.org/volto/contributing/index.html).
+- [ ] I succesfully ran [code linting checks](https://6.docs.plone.org/volto/contributing/linting.html) on my changes locally.
+- [ ] I succesfully ran [unit tests](https://6.docs.plone.org/volto/contributing/testing.html) on my changes locally.
+- [ ] I succesfully ran [acceptance tests](https://6.docs.plone.org/volto/contributing/acceptance-tests.html) on my changes locally.
+- [ ] If needed, I added new tests for my changes.
+- [ ] If needed, I added [documentation](https://6.docs.plone.org/volto/contributing/documentation.html#narrative-documentation) for my changes, either in the Storybook or narrative documentation.
+- [ ] I included a [change log entry](https://6.docs.plone.org/contributing/index.html#contributing-change-log-label) in my commits.
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