Document the use of `.editorconfig` to ensure proper UTF-8 detection by external text editors

**Your Godot version:**

4.x

**Issue description:**

See:

* godotengine/godot#27083
* godotengine/godot#96845
* godotengine/godot#97270

Once godotengine/godot#97270 is merged, the Visual Studio issue will no longer be fixed retroactively. Documenting the use of `.editorconfig` would increase the chances that users with existing projects or who uncheck the checkbox when creating a project will fix it when configuring an external text editor.

**URL to the documentation page (if already existing):**

* [Using an external text editor](https://docs.godotengine.org/en/stable/tutorials/editor/external_editor.html)
* [C# basics - Configuring an external editor - Visual Studio (Windows only)](https://docs.godotengine.org/en/stable/tutorials/scripting/c_sharp/c_sharp_basics.html#visual-studio-windows-only)