docs(examples): revamp things-advanced to use media handling

[mmustafasenoglu]

Summary

This PR revamps the things_advanced examples to showcase Falcon's built-in media handling instead of manual JSON serialization, as requested in #2309.

Changes

examples/things_advanced.py (WSGI) & examples/things_advanced_asgi.py (ASGI):

• Removed JSONTranslator middleware - replaced with direct usage of req.get_media() and resp.media
• Added TimingMiddleware - demonstrates req.context/resp.context usage by recording request processing time and setting Server-Timing header
• Fixed on_get() - now uses resp.media = result instead of resp.context.result
• Fixed on_post() - now uses req.get_media() instead of req.context.doc
• Fixed debug comment - "API" → "App" ("Useful for debugging problems in your App")
• Fixed ASGI Content-Length issue - removed the broken req.content_length == 0 check (moot with JSONTranslator removal)
• Enabled AuthMiddleware in ASGI example (was commented out in README.rst but present in source file)

README.rst:

• Synced embedded WSGI and ASGI code blocks with the updated example files
• Fixed all discrepancies: import order, parameter order, AuthMiddleware status, string formatting

Rationale

Per the issue discussion with @vytas7 and @CaselIT:

• JSONTranslator was removed altogether as it obscures Falcon's native media handling
• TimingMiddleware was added as a lightweight replacement that still demonstrates middleware context patterns
• The examples now properly showcase req.get_media() and resp.media as the recommended approach

Fixes #2309