docs: add axum integration example

[mrcrgl]

Summary

• add a new examples/axum.rs integration example that runs an axum server under ProcessManager
• demonstrate graceful shutdown and reload handling via runtime control messages
• add axum as a dev-dependency for example compilation
• document the new example in README and examples README
• include the new axum example in CI example execution
• update changelog entries for issue #11

Validation

• cargo fmt --all -- --check
• cargo clippy --all-features -- -D warnings
• cargo test --all
• cargo test --no-default-features
• timeout 30s cargo run --example simple
• timeout 45s cargo run --example dynamic_add
• timeout 30s cargo run --example restart_supervisor
• timeout 30s cargo run --example runtime_context
• timeout 30s cargo run --example axum

Closes #11