Add link to File API or the example code of the File API to the Save/Load tutorial

**Your Godot version:** 3.2.2, latest, stable

**Issue description:**
For a beginner Godot user who does not have extensive programming experience, the save/load tutorial is way to complicated and complex. To provide an easier way to learn save and loading (which are basic features of almost any game and therefore even novice beginner devs want those), I would think the tutorial section of the save/load documentation should start with a very basic example, then show a intermediate one and finally one showing complete/complex saving and loading.

As interim step, it would already help a lot to just add the File API example code at the top of the tutorial page, and/or at least link to the File API directly:
https://docs.godotengine.org/en/stable/classes/class_file.html#description

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/3.2/tutorials/io/saving_games.html