docs: expand HTTP caching guidance

[relaxfinger]

Summary

• explain memory, disk, and custom cache choices with practical scenarios
• document cache policies, HTTP cache headers, ETag revalidation, Vary variants, offline behavior, and cache lifecycle
• update the Demo to use a bounded disk cache with an explicit fallback TTL
• update English and Simplified Chinese READMEs and release notes

Validation

• swift test
• macOS Demo build
• iOS Demo build