Update hidden service website page with Unix socket info

We should update the https://www.torproject.org/docs/tor-hidden-service page with some information on how to setup an hidden service with Unix socket.

Since 0.2.6, both SocksPort and HiddenServicePort support Unix socket. Example:

HiddenServiceDir /your/HS/dir
HiddenServicePort 80 unix:/path/to/socket

For service that supports unix socket like nginx (http://nginx.org/en/docs/http/ngx_http_core_module.html#listen), you only need to point it to "/path/to/socket".