Document configuration properly (out of the README config-file table)

[polaris-is-online]

From PR #147 review (comment, README.md:124).

The per-file config table in the README is not the place for detailed per-field exposition. #147 trimmed the cache_warming detail back to sibling terseness, but we need a real home for config documentation.

Proposal

• Add a dedicated config reference (e.g. docs/config.md, or per-schema docs).
• Keep the README config-file table to one terse line per file.
• Document each field's meaning/economics where the detail actually belongs.