Normalize contribution docs with Toga

[freakboy3742]

What is the problem or limitation you are having?

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

Describe the solution you'd like

Replicate the sections of the Toga contribution guides that aren't Toga-specific in the Briefcase docs. For example, we don't need the discussion about core and backends; but we do need the section about not submitting from your main branch, and waiting for feedback.

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/toga#2453, beeware/rubicon-objc#427