Add hardening documention to warpgate docs

[krishardy]

I intend to add some documentation to warpgate-docs to describe how to harden it: set it up to run as a non-root user, rotate ssh and tls certificates, etc.

Issues aren't turned on for warpgate-docs, so I'm just adding this note here to open a communication channel before I start on the PR.

[krishardy]

serviced file: #1221