review git & github preparation

[nehamoopen]

Section 7.1.4.3 doesn't have code snippets and redirects to the GitHub Docs, we should update it because:

• people sometimes miss it because they're just looking for code snippets
• the guidance for Windows on the GitHub Docs suggests starting the ssh-agent using admin-elevated PowerShell, but since we're using Git Bash - the (more straightforward) commands for MacOS/Linux work fine.

Start the SSH agent:

eval "$(ssh-agent -s)"

Add private SSH key to ssh-agent:

ssh-add ~/.ssh/id_ed25519

[nehamoopen]

It would be good to review the whole preparation chapter. We could potentially incorporate callout blocks to provide a brief explanation of what the students are actually doing / dealing with at a given step, kind of like what we would say if we were taking someone through the steps in person.