Polish dimensionality demo unit assignment

[ss2098]

This follow-up to #261 updates docs/examples/Dimensionality-Demo.py to use the public unit-aware .array interface for dimensional assignment.

The previous fix made the demo run by manually writing non-dimensional values into .data. This polish assigns dimensional quantities through .array, for example:

T.array[...] = uw.quantity(1300.0, "kelvin")

and then shows that .data contains the corresponding non-dimensional internal storage.

This better demonstrates the intended units interface:

• .array for dimensional/unit-aware values
• .data for non-dimensional internal values

No solver behavior is changed.

Tested with:

pixi run python docs/examples/Dimensionality-Demo.py

Result: script completes successfully.

[lmoresi]

This is exactly the polish I suggested on #261 — .array[...] = uw.quantity(value, unit) dimensional assignment instead of manual scaling, plus the .array (dimensional) vs .data (ND) explanation. Verified it runs end-to-end (✅ Dimensional analysis complete). The demo now actually showcases the units interface. Thanks @ss2098!