Consider merging the tunneling docs

[joeyparrish]

Each backend has a separate document explaining how to set up and use SSH tunnels to talk to a device on another network. This is something we have had to do often for our own test lab, which is on a separate network from where we do our development.

However, most of the content in these tunneling docs is the same across the various Generic WebDriver Server backends. We should explore merging these docs and producing a general doc that covers all backends.