Add backend statement and Examples section to cost_distance docstring

[Melissari1997]

Closes #3587.

• Adds backend support paragraph (NumPy, CuPy, Dask+NumPy, Dask+CuPy)
• Adds a .. sourcecode:: python Examples block with a 3x3 uniform-friction example
• Adds a pin test verifying the backend sentence and a test that the example output matches the implementation
• No behaviour changes

Test plan

• test_docstring_states_all_backends -- passes
• test_docstring_example_matches_output -- passes
• Full test_cost_distance.py suite: 79 passed, 12 skipped (GPU-only)

[Melissari1997]

PR Review: Add backend statement and Examples section to cost_distance docstring

Blockers

None.

Suggestions

None.

Nits

None.

What looks good

• Backend statement matches the phrasing used by proximity, allocation, and direction in proximity.py, and accurately reflects the four backends (numpy, cupy, dask+numpy, dask+cupy) already dispatched in cost_distance.
• Examples section uses the .. sourcecode:: python convention, includes runnable >>> blocks with expected output, and is verified by the pin test test_docstring_example_matches_output so the printed output cannot drift.
• Pin test test_docstring_states_all_backends follows the same pattern as the existing tests in test_proximity.py (from issue #3091) and guards against the old wording coming back.
• The new tests pass: both docstring pin and example-output tests succeed alongside the full test_cost_distance.py suite (79 passed, 12 skipped).
• No behaviour changes, no dispatch or algorithm modifications -- pure doc and test addition.

Checklist

• Algorithm matches reference/paper — N/A (no algorithm change)
• All implemented backends produce consistent results — verified by existing tests
• NaN handling is correct — N/A
• Edge cases are covered by tests — existing coverage is adequate
• Dask chunk boundaries handled correctly — N/A
• No premature materialization or unnecessary copies — N/A
• Benchmark exists or is not needed — N/A (doc-only)
• README feature matrix updated — N/A (no new function)
• Docstrings present and accurate — verified

[brendancol]

LGTM