Document the various ways to connect to running applications

[jsierles]

People want to debug and poke at their apps in different ways. Bringing it all together in a single document can help us with support.

Stuff to include:

• fly ssh: running commands and selecting individual VMs
• fly proxy: for connecting directly to a Postgres instance or web service by linking it to a local port
• fly wg + fly ips private: for getting access to multiple VMs, for scripting to download files from all VMs or run commands across all VMs