Improve PR template to encourage appropriate context in descriptions

[alice-i-cecile]

Objective

• the existing PR template was repetitive
  • merge Objective and Solution sections
  • remove Changelog section
• the existing PR template often caused contributors to open PRs without adequate context
  • add a section on Context, with additional prompts for useful information
• the PR template did not make clear that authors should call out the areas that are tricky or controversial in their PR description
  • add a Feedback Wanted section

Context

• This template was first added in [Merged by Bors] - Add PR Template #2272.
• Bevy uses a collaborative review process, where anyone can review and merge PRs, as discussed in CONTRIBUTING.md.
• After talking with inexperienced contributors, I found that inadequate context on pull requests was the single most serious barrier to contributing, especially for PR reviews (which we are currently bottle-necked on).
• In many simple PRs, Objective and Solution were tediously repetitive, as seen in [Merged by Bors] - Stop labeling PRs with Needs-Triage #4686 for example.
  • More experienced authors would often merge these sections already as in [Merged by Bors] - update xshell to 0.2 #4789.
• I've made similar changes over at the stretch2 library, which I also maintain.

Feedback wanted

1. Do you have any other ideas on ways we can encourage contributors to add more helpful context to their PRs?
2. Is there a clearer way to mark sections as optional?
3. Does this appropriately balance ease-of-making-a-PR with ease-of-reviewing-PRs?
4. Are there any sections that can be safely cut? Can I make this even shorter?

[DJMcNab]

This doesn't follow the current PR template :)

[MrGVSV]

While fixing up the templates again, is it worth turning the suggestions into HTML comments? This would allow contributors to keep each suggestion so they don't miss anything and have something to reference.

But maybe that's just overkill/not as readable 😅

[alice-i-cecile]

But maybe that's just overkill/not as readable 😅

I think the readability argument is critical here: most of the tools (like Github's PR tool or Gitkraken) won't render the HTML as you're editing, which makes that style of changes relatively confusing to users.

[mockersf]

This is the size of the box when opening a PR:

In my opinion, everything that is out of this box is too much

[Vrixyz]

Mostly minor nitpicks