Revisions to all GitHub content

[brian-rose]

In our push to get the Intro to GitHub content created, we may have left some inconsistencies or differences in style among the sections. Once all sections are merged in, the final step should be to read through everything in order and see what needs ironing out.

[brian-rose]

We've set a meeting for Friday 2/25/2022 at 4:30 EST / 2:30 MST for a small group discussion (with me, @clyne, @jukent and @ktyle) to plan out the necessary revisions.

[jukent]

I am going to work on the branches/PR overlap today.

[jukent]

I'm reading through the forking, branching, and PR content more thoroughly to try and synthesize it better, and I think that "GitHub workflows" (currently covered in the PR chapter) should have its own chapter.