Refactor SSH instructions

Separate out the bits about ~/.ssh/config configuration from the VS Code docs into another document that also explains setting up ssh-agent.

In Windows: As Admin in Powershell: Get-Service ssh-agent | Set-Service -StartupType Automatic -PassThru | Start-Service

In macOS only there is an UseKeychain yes directive.

Suggest also using ForwardAgent=yes to allow ssh to nodes with jobs and to connect to other servers they may have access to.

Also somewhere suggest srun --jobid X --pty /bin/bash as an alternative to configuring ssh.