Add game channel admin API reference

[tudddorrr]

New game channel admin API docs

• Added a reference page covering the game channel admin API, including endpoints to create, update, delete and list channels and read their storage props
• Documented the related types: GameChannel, Prop, GameChannelStorageProp, PlayerAlias and RejectedProp
• Explained that props failing size or profanity checks return 400 with errors.props and a rejectedProps array

Navigation and collection updates

• Registered the new page in the admin meta.json so it appears in the Admin API reference
• Updated the Bruno collection structure description to include Game channels requests

Game config docs cleanup

• Removed the redundant scope explanation from the game config admin API overview