docs: add async SQLAlchemy + HNSW full example

[ChunkyTortoise]

Summary

The README documents async driver registration (Psycopg 3 register_vector_async) and HNSW index creation separately, but there's no combined example showing the end-to-end async pattern that production FastAPI/async Python applications typically use.

This adds a complete working example under "Async Full Example" in the SQLAlchemy section showing:

• Async engine + session setup (async_sessionmaker)
• Extension creation + table setup (run_sync)
• HNSW index creation with vector_cosine_ops
• Async vector insertion
• Async cosine distance nearest-neighbor query

Motivation

I use this exact pattern in a production document extraction API (docextract) with 768-dim Gemini embeddings, and had to piece together the async + HNSW workflow from separate README sections and test files. A combined example would have saved significant time.

Test plan

• Example is syntactically valid Python
• Follows existing README conventions (single quotes, same model pattern)
• Placed after async driver registration, before SQLModel section

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