docs(integrations): add Database Memory Service (#1662)

* docs(integrations): add Database Memory Service

Add a new integration entry for adk-database-memory, a drop-in durable
BaseMemoryService for the Python ADK backed by any async SQLAlchemy
dialect (SQLite, PostgreSQL, MySQL/MariaDB).

Repo: https://github.com/anmolg1997/adk-database-memory
Package: adk-database-memory on PyPI (Apache 2.0)

This fills the gap between InMemoryMemoryService (ephemeral, for dev)
and VertexAiMemoryBankService (managed GCP) for self-hosted durable
memory on the database the user already runs.

* Update formatting and wording

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Co-authored-by: Kristopher Overholt <koverholt@google.com>

Author: anmolg1997