github: Add a zulip/zulip style pull request template.

.github/pull_request_template.md

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+<!-- Describe your pull request here.-->
+
+Fixes: <!-- Issue link, or clear description.-->
+
+<!-- If the PR changes output, always include screenshots or code blocks to demonstrate your changes. If it seems helpful, add a screen capture of the new functionality as well.
+
+Tooling tips: https://zulip.readthedocs.io/en/latest/tutorials/screenshot-and-gif-software.html
+-->
+
+**Screenshots and how this was tested:**
+
+<details>
+<summary>Self-review checklist</summary>
+
+<!-- Prior to submitting a PR, follow our step-by-step guide to review your own code:
+https://zulip.readthedocs.io/en/latest/contributing/code-reviewing.html#how-to-review-code -->
+
+<!-- Once you create the PR, check off all the steps below that you have completed.
+If any of these steps are not relevant or you have not completed, leave them unchecked.-->
+
+- [ ] [Self-reviewed](https://zulip.readthedocs.io/en/latest/contributing/code-reviewing.html#how-to-review-code) the changes for clarity and maintainability
+      (variable names, code reuse, readability, etc.).
+
+Communicate decisions, questions, and potential concerns.
+
+- [ ] Explains differences from previous plans (e.g., issue description).
+- [ ] Highlights technical choices and bugs encountered.
+- [ ] Calls out remaining decisions and concerns.
+- [ ] Automated tests verify logic where appropriate.
+
+Individual commits are ready for review (see [commit discipline](https://zulip.readthedocs.io/en/latest/contributing/commit-discipline.html)).
+
+- [ ] Each commit is a coherent idea.
+- [ ] Commit message(s) explain reasoning and motivation for changes.
+
+Completed manual review and testing of the following:
+
+- [ ] Visual appearance of the changes.
+- [ ] Responsiveness and internationalization.
+- [ ] Strings and tooltips.
+- [ ] End-to-end functionality of buttons, interactions and flows.
+- [ ] Corner cases, error conditions, and easily imagined bugs.
+</details>