Add information on basic principles of composition in FSD

[illright]

Previously, the "Isolation -> Low Coupling & High Cohesion" page in the Reference section contained advice on how to maintain low coupling by composing either through passing JSX as props or by combining several slices from a layer in a slice above them. This is more of a guide than a reference, and it probably belongs in the "Cross-imports" guide, although it might need a better name