chore: Update the PR template

[upbqdn]

Motivation & Solution

I was going to edit the PR author's checklist so it mentions the changelog summary, and I ended up rewriting the whole PR template.

PR Author Checklist

Check before marking the PR as ready for review:

• Will the PR name make sense to users?
• Does the PR have a priority label?
• Is the documentation up to date?

Reviewer Checklist

Check before approving the PR:

• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
  PR blockers can be dealt with in new tickets or PRs.

And check the PR Author checklist is complete.

Follow Up Work

I will edit the release process docs to reflect the new changes around editing the changelog.