Add a completed project of the "Animating/Controlling thousands of fish with MultiMeshInstance" tutorial for the readers to download.

**Your Godot version:**
3.2.1 stable

**Issue description:**
There are multiple places where it's hard to follow the tutorial, adding a link to a completed project for the tutorial like there is for the FPS tutorial would let the readers find the solution to any issues that arise by themselves.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/3d/vertex_animation/animating_thousands_of_fish.html
https://docs.godotengine.org/en/stable/tutorials/3d/vertex_animation/controlling_thousands_of_fish.html