Add game stat admin API docs

[tudddorrr]

New Admin API documentation

• Added authentication guide explaining the Admin API uses admin API keys (scoped to games) for server-side management, distinct from the player-facing HTTP API.
• Created game stat admin API reference documenting endpoints for programmatic stat management (create, update, delete, reset) with optional metrics queries.

Documentation structure

• Added admin section to sidebar navigation with key icon and organized pages (authentication, game-stat-api).
• Included Admin API card on documentation homepage for discoverability.