docs(QuantumMechanics): add Hilbert space API map

[gloges]

Begins an API map for Hilbert spaces on Space d.

This covers the L²-space SpaceDHilbertSpace, its important subspaces, and operators acting on it. About half of the included items are already completed, while the rest represent a basic "wish list" for some missing subspaces and operators which should serve useful in the future.

[github-actions]

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