docs: add PR template

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

.github/pull_request_template.md

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+<!--
+Please give the pull request a short but descriptive title.
+Use [conventional commits](https://www.conventionalcommits.org/) to separate and summarize commits logically.
+
+Use this template as a guide. Omit sections that don't apply. You may link to information rather than copy it.
+More details about the template are at https://github.com/edx/open-edx-proposals/pull/180
+(link will be updated when that document merges)
+-->
+
+## Description
+
+Describe what this pull request changes, and why. Include implications for people using this change.
+Design decisions and their rationales should be documented in the repo (docstring / ADR), per
+[OEP-19](https://open-edx-proposals.readthedocs.io/en/latest/oep-0019-bp-developer-documentation.html), and can be
+linked here.
+
+Useful information to include:
+- Which edX user roles will this change impact? Common user roles are "Learner", "Course Author",
+"Developer", and "Operator".
+- Include screenshots for changes to the UI (ideally, both "before" and "after" screenshots, if applicable).
+- Provide links to the description of corresponding configuration changes. Remember to correctly annotate these
+changes.
+
+## Supporting information
+
+Link to other information about the change, such as Jira issues, GitHub issues, or Discourse discussions.
+Be sure to check they are publicly readable, or if not, repeat the information here.
+
+## Testing instructions
+
+Please provide detailed step-by-step instructions for testing this change.
+
+## Deadline
+
+"None" if there's no rush, or provide a specific date or event (and reason) if there is one.
+
+## Other information
+
+Include anything else that will help reviewers and consumers understand the change.
+- Does this change depend on other changes elsewhere?
+- Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.