Android Plugin development should be documented

**Your Godot version:** 3.4

**Issue description:** The documentation currently does not explain how an Android plugin can expose functionality for the Godot code to use, nor how to pass data between Godot and the Android plugin, nor any of the classes and methods available in godot-lib.aar.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/tutorials/platform/android/android_plugin.html