Add information about 3D asset direction conventions to "Importing 3D scenes"

[aaronfranke]

This PR adds a note to the documentation to explain the different conventions and specifications about orienting 3D assets in Godot. It explains Godot/OpenGL camera directions, Godot/Blender/glTF asset directions, and Godot/Blender cardinal directions. It links to the glTF specification for +Z being codified as asset front. I tried to be careful where I use the words "forward" vs "front" to make things clear.

Why not open this PR earlier: Before we were not certain what would end up in the engine, but now it is fairly certain so now seems like a good time. For example initially there was some push from @TokageItLab to add GLTF_* constants and document how glTF works in the core engine. I fully agree that this information must be documented, I just disagreed on it being in the core engine's constants and documentation, so now it can live in the docs repo :)

I'm fully open to feedback on any ways we can make this documentation more clear, concise, and comprehensive. I think we can all agree that coordinate systems are a confusing topic.

The third paragraph in this PR refers to the use_model_front option and the MODEL_* constants, so it expects that godotengine/godot#75875 and/or godotengine/godot#72842 is merged into the engine.

[TokageItLab]

I thinks it's fine. However, as mentioned in godotengine/godot#75228, there is often confusion in the handling of coordinates in Blender, so I would like to add another troubleshooting page separate from this.

[skyace65]

This part is confusing me (and making me realize my 3D teachers in college taught even less than I thought, which wasn't much). What exactly is "the camera" that you're referring to here? I assume it's not one you would attach to a player character

[aaronfranke]

@skyace65 Does this image help you understand?

[TokageItLab]

It may be paraphrased as a viewpoint of the viewport.

[skyace65]

So the camera in this case would be the viewport camera when you a open a blank scene?

[TokageItLab]

In most cases, yes.

[mhilbrunner]

@aaronfranke Any part of this that still needs to be discussed? (i.e., the discussion label) - it seems like a good improvement from my POV