Fix dimensionality demo for strict units

[ss2098]

This updates docs/examples/Dimensionality-Demo.py so that it runs with the current strict-units behavior.

The example previously failed because it:

1. created unit-bearing MeshVariable objects before reference quantities were set,
2. assigned plain dimensional values directly into .array, and
3. used older .to_nd(), .from_nd(), and UWQuantity reference-scaling calls that are not available in the current API.

The updated example keeps the dimensionality-tracking demonstration, but uses the current strict-units pattern:

• set reference quantities before creating unit-bearing variables,
• write non-dimensional values into .data,
• use scaling coefficients for dimensional/non-dimensional conversion.

No solver behavior is changed.

Tested with:

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

Result: script completes successfully.