Simplify the PR template

[hombit]

I think the current PR template puts a lot of pressure on potential contributors. Correct me if I'm wrong, but I have also seen a lot of PRs that actually use all of it, from both the LINCC Frameworks team and external collaborators. A few of the most recent PRs from the projects I track:
astronomy-commons/lsdb#995 (not used)
astronomy-commons/hats#527 (second half not used)
astronomy-commons/hats#506 (not used)
astronomy-commons/hats#514 (second half not used)
lincc-frameworks/nested-pandas#314 (actually used)

I think we should make it much shorter.

[drewoldag]

Personally I'm 100% onboard with shortening and simplifying the PR template. When I'm adding the PPT to a new or existing project, cutting out most/all of the PR template is almost always one of my first steps.

[hombit]

@drewoldag sounds reasonable, feel free to close!