add docs for testing with a database dependency

Created by: camdencheek

This PR adds some docs for my take on current best practices for testing with a database dependency. I think the dbmock stores have matured enough that it's worth sharing the new pattern with others, and while I'm at it, I also added a section that explains dbtest.NewDB(), with some clarification on when to use each package.

As always, I'm very open to suggestions -- especially on opinionated docs like testing best practices 😄

I plan to make a progress announcement soon that will include a link to these docs.