feat(github): Review the PR template

[chouetz]

What does this PR do?

Change the PR template.

Motivation

There are a lot of comments in the current template and several of them seem unnecessary to me (they just describe with more words the current section name). It is a drawback when updating the PR description as we are polluted with this comments.

Additional Notes

I took the opportunity to reschuffle the sections, setting test and drawbacks/trade-offs sections before the additional notes which is an appendix section according to me (and I let a comment for this one only)

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes