Fix unit-aware coordinate evaluation example

[ss2098]

This updates docs/examples/example_unit_aware_coordinates.py so that it runs with the current evaluate() behavior.

The example previously failed because it:

1. assigned mesh vertex coordinates to a degree=2 MeshVariable, causing a shape mismatch, and
2. passed a Python list of UWQuantity coordinate tuples directly to uw.function.evaluate(), which is not accepted by the current non-dimensionalisation path.

The updated example:

• initializes the degree=2 MeshVariable using its own coordinates,
• converts unit-aware coordinate values explicitly to SI base units,
• passes numeric coordinate arrays to uw.function.evaluate(),
• preserves the original purpose of demonstrating coordinate evaluation with unit-aware inputs.

No solver behavior is changed.

Tested with:

pixi run python docs/examples/example_unit_aware_coordinates.py

Result: script completes successfully.