Add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+### PR creator: Description
+
+Describe the overall goals of this pull request.
+
+
+### PR creator: Are there any relevant issues/feature requests?
+
+* GitHub Issue #...
+* Jira Issue #...
+
+
+### PR creator: Which product versions do the changes apply to?
+
+When opening a PR, check all versions of the documentation that your PR applies to.
+
+- [ ] SLL 9 *(current `main`, no backport necessary)*
+- [ ] SLL 8
+- [ ] SLL 7
+
+### PR reviewer only: Have all backports been applied?
+
+The doc team member merging your PR will take care of backporting to older documents.
+When opening a PR, do *not* set the following check box.
+
+- [ ] all necessary backports are done