Improve documentation for exporting a gdnative project for macOS

**Your Godot version:**

3.4+

**Issue description:**

The documentation for exporting a Godot C++ gdnative project for MacOS with its compiled gdnative .dylib file (plus external dylib files ie to include inside `Frameworks`) to make an .app is not enough beginner friendly. Current explanations are too complex and missing examples for a none-developer Mac. In addition following https://github.com/godotengine/godot/pull/56047 the `add_osx_plugin_file` is not described in official documentation and not showed from the exporter GUI.

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

https://docs.godotengine.org/en/stable/tutorials/export/exporting_for_macos.html