docs(csharp): document common server runtime functions

[alanparesys]

This PR expands the C# server runtime documentation.

• Adds a “Runtime-specific functions” section to the C# server functions page
• Documents how to use EventHandlers for built-in and custom events
• Adds examples for TriggerClientEvent (broadcast and single player)
• Shows how to trigger server events from server-side code
• Provides a simple HTTP example using HttpClient
• Adds a small listplayers command example using Players

The goal is to bring the C# server docs closer to the existing Lua/JavaScript runtime documentation so C# users don’t
have to guess how to translate common patterns.