docs: add PULL_REQUEST_TEMPLATE.md for consistent PR descriptions

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for contributing a pull request! Here are a few tips to help you:
+
+1. If this is your first contribution, make sure you've read the Contribution Checklist https://developers.mattermost.com/contribute/getting-started/contribution-checklist/
+2. Read our blog post about "Submitting Great PRs" https://developers.mattermost.com/blog/2019-01-24-submitting-great-prs
+3. Take a look at other repository specific documentation at https://developers.mattermost.com/contribute
+-->
+
+#### Summary
+<!--
+A clear, concise description of what this pull request does and why it is needed.
+-->
+
+#### Changes
+<!--
+Bullet list of changes. Be specific about the files, hooks, components, or APIs affected (server/webapp/build/CI).
+-->
+
+#### Ticket Link
+<!--
+Link the relevant GitHub issue or JIRA ticket, e.g.
+
+  Fixes https://github.com/mattermost/mattermost-plugin-demo/issues/XXXXX
+
+If there is no related ticket, leave this section empty.
+-->
+
+#### Release Notes
+<!--
+For user-facing changes include a release note using the fenced block:
+
+```release-note
+Description of the change for the release notes.
+```
+
+For non-user-facing changes (refactors, tests, docs, CI), use:
+
+```release-note-none
+```
+-->
+
+#### Screenshots
+<!--
+If the change affects the UI or any visible behavior, attach screenshots or GIFs.
+-->
+
+#### Testing
+<!--
+Describe how this was tested. Include manual steps and/or the automated tests added or updated.
+-->
+
+#### Checklist
+- [ ] Changes follow the project's code style guidelines
+- [ ] Self-review completed
+- [ ] Documentation updated (if applicable)
+- [ ] New tests added / existing tests updated (if applicable)
+- [ ] All existing tests pass (`make test` / `make test-server` / `make test-webapp`)
+- [ ] Release notes included (or `release-note-none`)