Move SSH Key lesson from Git to Extras

Related with #691.
The default is to use HTTPS. Here is my suggested wording for the beginning of this paragraph:

We use HTTPS here because it does not require additional configuration. Unfortunately this option also requires you to type your username and password every time you interact with GitHub. After the bootcamp, we recommend that you set up SSH keys, which will remove the username and password requirement, by following one of the great tutorials from...