docs: add a PR checklist template (#4564)
Manual testing of this template can be done with the new playground created for that purpose, see https://code.forgejo.org/forgejo/pr-and-issue-templates/pulls/19.
![image](/attachments/1ee36ae1-669f-47d8-8307-9734faa0dc2a)
## Testing instructions
* Fork https://code.forgejo.org/forgejo/pr-and-issue-templates
* Create a pull request against https://code.forgejo.org/forgejo/pr-and-issue-templates
* See that the commit message is on top and the checklist below it
---
Use cases:
* https://codeberg.org/forgejo/forgejo/pulls/4553
* https://codeberg.org/forgejo/forgejo/pulls/4554
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4564
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: twenty-panda <twenty-panda@noreply.codeberg.org>
Reviewed-by: thefox <thefox@noreply.codeberg.org>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>