docs: add hard Sudoku example

Transcribe the hard 9x9 Sudoku puzzle from the provided image into the
plain-text example format accepted by the solver. The new example gives users a
ready-made input file for manual CLI checks.

Document the example directory in the README project decomposition and include a
command showing how to run the solver against this puzzle.

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

Refs: none

sudoku-examples/hard.sudoku

@@ -0,0 +1,9 @@
+1 0 0 0 3 4 0 0 8
+0 7 0 6 8 0 0 3 0
+0 0 8 2 1 0 7 0 4
+0 5 4 0 9 0 6 8 0
+9 1 0 5 0 8 0 2 0
+0 8 0 3 0 0 0 0 5
+3 0 5 9 0 6 8 7 1
+0 0 6 0 0 0 0 4 0
+0 0 1 0 7 0 2 0 0