Update setup.md

[hollandjg]

Add links to instructions on:

• [x] Installing python
• [x] Setting up Git
• [x] Setting up GitHub
• [x] Setting up a PyPI account
• Add a link to Software Carpentry Version Control with Git – decided not to do this any more – there's a whole session on Git.

(see https://github.com/INTERSECT-training/Documentation/blob/main/setup.md for an example)