Document preflight training requirements

[sebbacon]

We'll want to link new external users to some page in the documentation that walks them through all the things we require them to do before we kick off their support period.

Much of this is already in the docs, but we want some kind of checklist or something.

Something along these lines:

• Complete some kind of git tutorial?
• Install Github Desktop, Docker (not version 3 I think! @HelenCEBM and @esnightingale both had issues with the latest one), Python
• Install a decent text editor with syntax highlighting for python and yaml joy
• Install opensafely-cli and successfully run opensafely --version
• Clone the repo the OpenSAFELY team will have set up for you
• Run opensafely run run_all and observe the output
• Somehow tweak the project.yaml and perhaps the default python script to do something different, and run that locally
• Push the change to your repo and observe the tests passing
• Access our Level 4 server (optional)