docs(csgo): explain appid vs clientappid for the standalone CS:GO client

[dgibbs64]

Summary

Documents the CS:GO App ID change in GameServerManagers/LinuxGSM#4963 (v26.3.0). Please merge only after #4963 is merged, because it describes the new clientappid setting.

game-servers/counter-strike-global-offensive.md: new Client App ID section (anchor #client-app-id, linked from the csgoserver _default.cfg comment) covering:

• appid (740, the SteamCMD download) vs clientappid (4465480, the App ID players connect with and the GSLT must match)
• a table for choosing clientappid: standalone CS:GO uses 4465480, CS2 csgo_legacy uses 730
• one client type per server; two separate installs to serve both
• LinuxGSM rewriting steam_appid.txt and steam.inf every start, with clientappid="" to opt out
• NoLobbyReservation, which standalone clients require, installed via mods-install
• an Upgrading warning for existing servers: new GSLT plus NoLobbyReservation, or set clientappid="730"

steamcmd/gslt.md: the hint said "730 for CS:GO". It now says 730 is CS2 and points CS:GO admins to clientappid.