add new extra lesson about forking

Add the first draft of a new lesson about forking. The lesson covers creating a fork, creating pull requests, and reasons for using a fork instead of working off of one repo. Leaves out the details of code review.

This encompasses most of the material that is in lesson 03. The plan is to rewrite 03 to focus on pull requests and code review. It should cover using pull requests from different branches in the same repo as well as between forks.