Write a "Developer Setup" guide for onboarding

Summary: Fixes T11561. Collect guidance about local configuration which hasn't been obvious in the past.

Test Plan:

• Read document carefully.
• Used ./bin/diviner generate to generate documentation.
• Previewed in Diviner locally:

{F4795021}

Reviewers: amckinley, chad

Reviewed By: chad

Subscribers: cspeckmim

Maniphest Tasks: T11561

Differential Revision: https://secure.phabricator.com/D17641