Move SSH Key lesson from Git to Extras

Close #691.

After some discussion about have and where to put the lesson that
cover SSH Key I and @wking agree to move it to
`novice/extras/06-sshkeys.md` and just add a note + links to help
students set up their own SSH Key for the Git host they will be using.

This commit includes @jdblischak wording about HTTPS vs. SSH.

This commit also avoid non-portable instructions to copy and paste
the public key.