Add a tutorial page on using ResourcePreloader

**Your Godot version:** 3.3.3

**Issue description:** Godot has a seldom-known node called ResourcePreloader. It currently has no tutorial and its class reference page is fairly sparse. We should create a dedicated tutorial page as this node sports its own bottom panel editor when selected (therefore, it needs screenshots).

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/classes/class_resourcepreloader.html