Update README for current v4 behavior

[Kralizek]

Summary

Refresh both consumer-facing READMEs after the v4 configuration and JSON stream provider changes.

• fix the AddObjectAsFallback examples so they reflect providers already registered by WebApplication.CreateBuilder
• clarify that fallback registration is inserted at the lowest precedence even when called after existing providers
• document multi-level root section paths such as Features:Payments
• remove stale wording about a shared custom flattening implementation
• tighten supported-shape wording around object graphs
• simplify prerelease wording to consumer-relevant information
• align the package README with the repository README and current public behavior