Improve ArrayMesh examples by adding a rectangle example

[MIfoodie]

Your Godot version: 4.2.1.stable

Issue description:
I believe that it will be beneficial to include an example on how to generate a simpler shape, such as a rectangle, before providing the example on how to generate the sphere. This would help because it would make the example code smaller to show the "bare bones" of how ArrayMesh really works.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/3d/procedural_geometry/arraymesh.html

[tetrapod00]

For anyone who is interested in starting contributing with this issue:

• The existing example for a sphere should give you enough context on how the API works.
• A rectangle or quad is two triangles that share four vertices between them
• We have docs on how to get started contributing here