Normalize contribution docs with Briefcase/Toga

[freakboy3742]

What is the problem or limitation you are having?

Rubicon has a contribution guide for docs and code; so do Toga and Briefcase. However, there are some elements of the Toga and Briefcase docs that would be useful to include in the Rubicon docs.

Describe the solution you'd like

Replicate the sections of the Toga and Briefcase contribution guides that aren't Briefcase- or Toga-specific in the Rubicon docs. The one section I've noticed in particular is the discussion about running CI checks locally as part of the PR submission process; there may be others.

Describe alternatives you've considered

Consolidate these docs upstream (on the beeware.org website) so that the content isn't duplicated unnecessarily. However, developing a good narrative flow with an external site like that may be complex.

Additional context

See also beeware/briefcase#1696, beeware/toga#2453.

[kattni]

This is in-progress. The first step is getting the Briefcase and Toga contribution docs synced. Once that layout is approved, the next step is updating Rubicon's contribution guides.