Tighten machine modeling guides

[SandroMaglione]

Summary

• Replace the model-facing API catalogue with an example-led statechart modeling guide.
• Add a focused guide for Effect Atom and React machine ownership patterns.
• Document direct Machine.make({...}).handle({...}) chaining as the default authoring shape.
• Render docs/agent-guide.md on the documentation website and place it between Changelog and Machine API in the navigation.

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required

No example package changed. The local pnpm perf:runtime smoke test passed; the documentation-only CI classification does not require a base-versus-PR performance comparison.