[DO NOT MERGE] docs: add pod domain example for Phoenix

[mikehostetler]

Summary

• add a top-level examples area
• add a Phoenix-shaped Pod domain example that wraps a Pod in a public domain module
• demonstrate the convention of a domain API module with nested Pod, policy, artifacts, agents, actions, plugins, and sensors

Notes

• this is intended for design/docs feedback
• not proposing new core runtime APIs in this PR
• example code lives outside the normal lib compile path on purpose