[TODO] Update Server install guide to include GSLT documentation when using sv_use_steam_networking

[brysondev]

Another addition to consider for the sv_use_steam_networking command, generating a GSLT token and setting it by doing +sv_setsteamaccount [GSLT] will set a persistent IP address.
https://steamcommunity.com/dev/managegameservers -> Game ID: 243750 (Source 2013 Multiplayer) and memo NTRE server 1 -> use generated token.

Originally posted by @brysondev in #959 (comment)

But of course, with our own AppID when we get one assigned.