Remove tutorials category from documentation

[ytdHuang]

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Description

This PR removes the Tutorials category in documentation.
I also re-organized a little bit of the cluster introduction page.

[albertomercurio]

Perhaps we should wait to have the tutorials repo first. At least for the examples already present here.

[ytdHuang]

Perhaps we should wait to have the tutorials repo first. At least for the examples already present here.

Yes, I'm also thinking about it.

I think we can merge this after moving low rank tutorial to the tutorials website : qutip/qutip-julia-tutorials#6