add git clone (and checkout) to diagram

[aculich]

This diagram could better explain the process starting from the initial git clone (and implicit checkout of the main branch):

I received confused questions about why the git checkout was part of this diagram in regards to committing and pushing to github, in particular because the diagram does not match the explanatory context around it, as the following steps 1-8 don't mention it at all, and then when branching is mentioned in collaboration the diagram isn't referenced again.

Here's a crude update that we can think about how to refactor: