PR template: Make it better!

[9999years]

The PR template is kinda unclear & could use some love.

Having a "part A" and "part B" is odd when they share items. We should have a "this PR changes behavior or interfaces" checkbox instead.

Automated comments for PRs that lack tests or release notes would be helpful.

[ffaf1]

We should have a "this PR changes behavior or interfaces" checkbox instead.

And then a sublist of checkboxes for things that apply only to behaviour-modifying PRs?

[ffaf1]

See #8511 for previous discussion. This pops out:

People keep complaining about our PR template in that it has a long set of checkboxes that are not necessarily relevant for small contributions, and those are usually made by newcomers, so they get especially confused. Maybe, we need another template: something like "Documentation" or "User-invisible change" without any checkboxes.