Add explanation of purpose and usecase between MeshInstance2D and Polygon2D in a 2D mesh workflow

**Your Godot version:** Godot 3.4.4, Godot

**Issue description:**
This tutorial about 2D Meshes in Godot is pretty good, however is does not mention why I would use a MeshInstance2D over a Polygon2D or vice-versa. Both seem to have a very similar workflow (using the converter in the Sprite toolbar), and I would assume similar benefits.

Also the graphic need to be updated. In particular this one:
https://docs.godotengine.org/en/latest/_images/mesh2d2.png

This is not a documentation issue, but I suppose the naming should be changed to "Convert to Meshinstance2D" instead of "Mesh2D", since the Mesh2D does not actually exist.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/2d/2d_meshes.html
https://docs.godotengine.org/en/latest/tutorials/2d/2d_meshes.html