docs: add extreme Sudoku example

Transcribe the extreme 9x9 Sudoku puzzle from the provided image into the
plain-text example format accepted by the solver. This gives users another
ready-made input for exercising sparse puzzles from the command line.

Update the README example commands so the new file is visible alongside the
existing hard puzzle.

Test Plan:
- cargo run -- sudoku-examples/extreme.sudoku

Refs: none

README.md

@@ -29,6 +29,7 @@ Example puzzles live in `sudoku-examples/`. For instance:
 
 ```sh
 cargo run -- sudoku-examples/hard.sudoku
+cargo run -- sudoku-examples/extreme.sudoku
 ```
 
 ## Project Decomposition