Revise intro topic lists and order

“Getting to know” feels more complete with a broader survey of GitHub
and Git than just one exclusively.

Moving the order of acquiring the code first, then focusing on branches
seems more logical so local and upstream branches can be discussed all
at once.
Add GitHub shortcuts

Foundations course serves as survey of GitHub features and should
include efficiency tips including these keyboard shortcuts.
Revise "commit" topic bullets
Add additional "Management" bullet topics
Add "Special Features" topics