Add an attempt at explaining the ranks and dimensions.

Wolthera van Hövellrequested to merge
Woltherav/gih_multi_dimensions into master

This documents this: https://invent.kde.org/kde/krita/commit/1bde9e70818945d35bbffd2af0e4d5a727035207

I tried to keep it simple, but it fundamentally is not.

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