docs: rework README and book intro for directory-aware schema

[edochi]

Summary

• README: replaced generic Quick Start with a minimal inline example (5 files, 3 directories) that demonstrates directory-aware inference, path-based validation, and search — the three "aha" moments
• Book introduction: added "Directory-aware schema" section (types, paths, nullability), "tightest fit" callout explaining allowed/required inference, added per-table columns row to the DB analogy table
• Book getting-started: added sensor_type toml example showing inferred path constraints, improved check section framing around path-awareness

Addresses TODO-0118.

Test plan

• mdbook build book/ passes
• Verify rendered book pages read well
• Verify README example is clear to a first-time reader