Improve structure of the repositories

[NathanLovato]

We updated this repo lately to make it easier to add the framework to your own games. The demos are now a separate project, https://github.com/GDQuest/godot-steering-ai-framework-demos

But:

• It's not convenient yet to clone this repository as a submodule, it comes with some useless files.
• Demos are hard to find.

We should clearly and prominently cross-link the two repositories, but also find a way to make installing the framework as efficient as possible.

@Razoric480 How about having this repo as the main one with demos, so people can test, and having a bare repository that's just for submodules instead? The idea of this main repo, that's the most starred, is to help people discover and get started with the framework. So having the demos would be welcome I think.

I also don't like too much that we'd have to maintain two repos and copy the framework to the second repo to do that. Not sure there's a way around that though.

[Razoric480]

Godot-steering-ai-framework
	changelog
	readme
	etc
	godot
		addons/com.gdquest.godot-steering-ai-framework
		src
			arrive
			arrive3D
			etc

gsai-submodule
	agents
	behaviors
	proximities
	gsaiagentlocation.gd
	etc

Something like this?

[NathanLovato]

I guess that would be it, yes. The submodule itself is going to be for the few persons who might use it that way, the rest of the content, which can also be used for learning purpose, would be more important I think, so we'd better group it all and document it well in one place.