docs: document cloud-core (#9)

This ports over the docs from
https://github.com/Incendo/cloud/tree/master/docs for cloud-core that
are still relevant in 2.0 and fill in the gaps where needed.

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:books: Documentation preview :books::
https://incendocloud--9.org.readthedocs.build/en/9/

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Co-authored-by: powercas_gamer <[email protected]>