Add contributing guide and pr checklist

.github/pull_request_template.md

@@ -3,13 +3,27 @@ Ensure your title is short, descriptive, and in the imperative mood (Fix X, Chan
 For a good inspiration of what to write in commit messages and PRs please review https://chris.beams.io/posts/git-commit/ and our https://jellyfin.org/docs/general/contributing/issues page.
 -->
 
-**Changes**
-<!-- Describe your changes here in 1-5 sentences. -->
+### Changes
+<!--
+Describe your changes here in 1-5 sentences.
+Please include before/after screenshots of any UI changes.
+-->
 
-**Issues**
-<!-- Tag any issues that this PR solves here.
-ex. Fixes # -->
+### Issues
+<!--
+Tag any issues that this PR solves here.
+ex. Fixes #
+-->
 
-**Code assistance**
-<!-- If code assistance was used, describe how it contributed
-e.g., code generated by LLM, explanation of code base, debugging guidance. -->
+### Code assistance
+<!--
+If code assistance was used, describe how it contributed
+e.g., code generated by LLM, explanation of code base, debugging guidance.
+-->
+
+---
+
+* [ ] I have read and followed the [contributing guidelines](https://github.com/jellyfin/jellyfin-web/blob/master/CONTRIBUTING.md).
+* [ ] I have tested these changes.
+* [ ] I have verified that this is not duplicating changes in an existing PR.
+* [ ] I have provided a *substantive* review of [another web PR](https://github.com/jellyfin/jellyfin-web/pulls?q=is%3Apr+is%3Aopen+-label%3Adependencies+-label%3Ablocked+-label%3Abackend+-label%3Ainvalid+-label%3A"merge+conflict"+-label%3Astale+-author%3A%40me).