docs: add empty Sudoku example

Add a fully blank 9x9 Sudoku input alongside the harder sample puzzles. This
fixture gives maintainers a simple way to exercise parser handling for all-zero
cells and to observe the solver's deterministic fill order when no givens are
present.

The example does not change solver behavior. It expands the documented example
set used for manual command-line checks.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo run -- sudoku-examples/empty.sudoku

Refs: none

sudoku-examples/empty.sudoku

@@ -0,0 +1,9 @@
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0
+0 0 0 0 0 0 0 0 0