Add Git authentication steps in setup

[isoyuki]

Raised during the first training session.

If the recruits are new to the GitHub platform, it might be good to add a note about git authentication, either using PAT or SSH (or link to the relevant documentation)

[jasparm]

There is an M3 specific explanation of using SSH with GitHub in section 3.1

[oraqlle]

We should add GitHub token generation instructions @MonashDeepNeuron/hpc-training.