docs: add Advanced-Alchemy to Thin Abstractions

[hasansezertasan]

Adds Advanced-Alchemy to the Thin Abstractions section.

Advanced Alchemy is a companion library for SQLAlchemy from the Litestar organization, providing:

• Sync and async repository and service layers to eliminate boilerplate CRUD
• Ready-to-use base models and mixins (UUID/BigInt primary keys, audit columns, slugs)
• Extra column types (encrypted strings, files, password hashes, GUIDs)
• Built-in Alembic migration tooling and a database management CLI
• Integrations for Litestar, FastAPI, Flask, Sanic, and Starlette

The entry follows the list's existing reStructuredText pattern (named reference in the body + link target at the end of the section).