PR template rework

[brian-smith-tcril]

Repos throughout the Open edX GH org have a variety of PR templates. These are often long, often ignored, and often include lists of checkboxes with steps that may not apply to any given PR. This isn't only an issue for PR authors, but also for reviewers, as a PR created by someone who doesn't fill in the template will just be a bunch of irrelevant boilerplate for a reviewer to parse.

We should instead use a shared template throughout the entire org that

todo

• Make an ADR for this in https://github.com/openedx/.github/tree/master/docs/decisions
• Make sure https://docs.openedx.org/en/latest/developers/quickstarts/so_you_want_to_contribute.html is accurate and up to date
  • Bonus: include a template people can copy here
• Update the template in this repo to be a comment linking to the updated doc
• Remove templates from all repos in the org so they use this one