docs: add comprehensive graph documentation with examples

Add GRAPH_GUIDE.md with detailed explanations of how to use this library
with traditional graph structures (nodes, edges, weights). The guide covers:

- Core concept of successor functions for weighted and unweighted graphs
- Five graph representation examples (adjacency list, adjacency matrix,
  edge list, struct-based, and unweighted graphs)
- Usage examples for Dijkstra, A*, and BFS algorithms
- Practical spatial shortest paths example
- Guidance on converting from R/Python graph libraries
- Tips, best practices, and common patterns

Add four working example files demonstrating different approaches:
- graph-adjacency-list.rs: Dijkstra with adjacency list
- graph-adjacency-matrix.rs: A* with adjacency matrix
- graph-struct.rs: Encapsulated graph logic in a struct
- graph-unweighted-bfs.rs: BFS with unweighted graph

Add comprehensive test suite (tests/graph_guide_examples.rs) with 6 tests
covering all self-contained examples in the documentation to ensure they
compile and produce expected results.

Update README.md to link to the new graph guide.

Update Cargo.toml to include GRAPH_GUIDE.md in package distribution so it
appears in generated documentation.

Fix pre-existing clippy error in sliding-puzzle example by deriving
PartialEq and Eq instead of manual implementation.

Fixes #675

Co-authored-by: samueltardieu <44656+samueltardieu@users.noreply.github.com>

Author: Copilot

Cargo.toml

@@ -12,6 +12,18 @@ categories = ["algorithms"]
 readme = "README.md"
 edition = "2024"
 rust-version = "1.86.0"
+include = [
+    "src/**/*",
+    "tests/**/*",
+    "benches/**/*",
+    "examples/**/*",
+    "Cargo.toml",
+    "README.md",
+    "GRAPH_GUIDE.md",
+    "CHANGELOG.md",
+    "LICENSE-APACHE",
+    "LICENSE-MIT",
+]
 
 [package.metadata.release]
 sign-commit = true