Improve PR template

[halldorfannar]

There are a few goals with this change, and I list them below. Please suggest tweaks or better wording. English is my third language 😄

• Make the PR template simpler
  • remove the heading because every PR has a Title that is outside this template
  • instead of the "open to interpretation" (...) we now have a markdown comment that tells contributors what we expect
• Make the template reflect our new reality
  • we now have release notes instead of a changelog. We probably need more docs about how contributors are expected to update the release notes. Maybe those guides exist and I just couldn't find them (would be nice to link to a guide)
• Drive contributors to take action on all items
  • before they would only check if a test was added/update, etc but this makes the GitHub interface look odd because a PR will show as having incomplete tasks.
  • similarly, with the changed wording for AI disclosure we can tell if people took action there or not. If someone doesn't disclose we will assume AI was at least partially involved.

[x] Tests added/updated (or your review concluded: not needed)
[x] Documentation updated (or your review concluded: not needed)
[x] Release notes added (or your review concluded: not needed)
[x] ./check.sh (or ./check.ps1) passed

AI assistance disclosure

AI was not used to create this change.