Improve documentation for exporting a gdnative project for macOS

[Lecrapouille]

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

[Calinou]

cc @bruvzg