Link each due diligience item in PR templates to documentation

[reefdog]

Feature Request

Is your feature request related to a problem? Please describe.

Currently, our PR template contains a checklist of due diligence items. For new contributors, it may not be clear what our expectations or best practices for each item are.

Describe the solution you'd like

We should document our guidance and expectations / best practices for each of these items, and then link the checklist items to that documentation in the template.

Describe alternatives you've considered

I hadn't really considered any. I suppose there's always not doing this.

Teachability, Documentation, Adoption, Migration Strategy

We have a CONTRIBUTING file that contains some of this information already; we should at least ensure guidance for each due diligence item is expressed there, and link to it. Some items, like TESTING, may justify their own separate documentation file.

[slifty]

Moving this to the meta repository since our PR templates live there now!