Add documentation for strongly typed signal emission in C#

**Your Godot version:** v4.7.1.stable.mono.official [a13da4feb]

**Issue description:** Godot C# has a way to emit signals with a fully-typed, generated method. It should be documented.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/latest/tutorials/scripting/c_sharp/c_sharp_signals.html#signal-emission

The syntax is `EmitSignal{SignalName}`: https://github.com/godotengine/godot/pull/97588

Start at the existing signal-emission section of c_sharp_signals.html and review the strongly typed emission syntax, EmitSignal{SignalName}. Read pull request 97588 for the relevant behavior, then document the syntax and verify that the page clearly explains how to use it.

Written by the indexing model from the issue text.