doc: Add a PR template

.github/pull_request_template.md

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+Related to #
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+Changes proposed in this pull request:
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+How to test the feature manually:
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+1.
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+Pull request checklist:
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+- [ ] code is manually tested
+- [ ] interface works on both mobiles and big screens
+- [ ] accessibility has been tested
+- [ ] tests are updated
+- [ ] French locale is synchronized
+- [ ] documentation is updated (including comments, commit messages, migration notes…)
+
+_If you think one of the item isn’t applicable to the PR, please check it
+anyway and precise `N/A` next to it._