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

[dalexeev]

Your Godot version:

4.x

Issue description:

See:

• Can't load .cs files containing "special" characters (like ç è ì) in comments or strings (encoded in CP-1252). godot#27083
• [Editor] Add .editorconfig to the projects. godot#96845
• Create .editorconfig file only on project creation 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
• C# basics - Configuring an external editor - Visual Studio (Windows only)