admin: add GitHub PR template. (#24365)

The template aims to ensure all PRs include the required
information for reviewers. The checklist items help ensure merging
happens quickly and in the correct manner.

Co-authored-by: Tim Gross <tgross@hashicorp.com>

.github/pull_request_template.md

@@ -0,0 +1,35 @@
+### Description
+<!-- Please describe why you're making this change and point out any important details the reviewers
+should be aware of.-->
+
+### Testing & Reproduction steps
+<!--
+* In the case of bugs, please describe how to reproduce it.
+* If any manual tests were done, document the steps and the conditions to reproduce them.
+-->
+
+### Links
+<!--
+Please include links to GitHub issues, documentation, or similar which is relevant to this PR. If
+this is a bug fix, please ensure related issues are linked so they will close when this PR is
+merged.
+-->
+
+### Contributor Checklist
+- [ ] **Changelog Entry** If this PR changes user-facing behavior, please generate and add a
+  changelog entry using the `make cl` command.
+- [ ] **Testing** Please add tests to cover any new functionality or to demonstrate bug fixes and
+  ensure regressions will be caught.
+- [ ] **Documentation** If the change impacts user-facing functionality such as the CLI, API, UI,
+  and job configuration, please update the  Nomad website documentation to reflect this. Refer to
+  the [website README](../website/README.md) for docs guidelines. Please also consider whether the
+  change requires notes within the [upgrade guide](../website/content/docs/upgrade/upgrade-specific.mdx).
+
+### Reviewer Checklist
+- [ ] **Backport Labels** Please add the correct backport labels as described by the internal
+  backporting document.
+- [ ] **Commit Type** Ensure the correct merge method is selected which should be "squash and merge"
+  in the majority of situations. The main exceptions are long-lived feature branches or merges where
+  history should be preserved.
+- [ ] **Enterprise PRs** If this is an enterprise only PR, please add any required changelog entry
+  within the public repository.