docs: improve cookbook and learning path

[Tuntii]

This PR enhances the RustAPI Cookbook and Learning Path as part of the scheduled maintenance.

Changes

1. New Recipes:

   • recipes/advanced_middleware.md: Covers Rate Limiting, Request Deduplication, and Response Caching using rustapi-extras.
   • recipes/audit_logging.md: Comprehensive guide to the Audit Logging system, including GDPR/SOC2 compliance features.
   • recipes/oauth2_client.md: Guide to implementing OAuth2 client flows (Google, GitHub, etc.).

2. Learning Path Update:

   • Added Module 8: Advanced Middleware.
   • Added Module 7: Authentication (expanded with OAuth2).
   • Added Module 12: Observability & Auditing.
   • Renumbered modules to maintain a logical progression.

3. Documentation Tracking:

   • Updated docs_inventory.md and docs_coverage.md to reflect the new documentation.
   • Generated run_report_2026-02-16.md.

These changes ensure the documentation keeps pace with the features available in rustapi-extras and provides a more complete learning experience for users.

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PR created automatically by Jules for task 13845243646235523051 started by @Tuntii

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[Tuntii]

lgtm