docs(graph): expand statum-graph usage guide

[eboody]

Summary

• expand the statum-graph README with Mermaid output, graph traversal, entry-point guidance, and scope notes
• document when to use MachineDoc::from_machine(...) versus MachineDoc::try_from_graph(...)
• clarify Mermaid renderer guarantees in the Rust docs

Verification

• cargo test -p statum-graph --offline
• RUSTDOCFLAGS='-D warnings' cargo doc -p statum-graph --offline --no-deps