Information about declaring multi-dimensional arrays on the array documentation page.

**Your Godot version:**
Godot 4.0
**Issue description:**
Information about declaring multi-dimensional arrays would be really useful.

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To declare multi-dimensional arrays:
var multi_dimensional_array= [[],[]]
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**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_array.html