The official documentation is at: http://docs.alfresco.com
What is Ingres ?
Ingres is the vendor of the Open Source Database System 'Ingres Database' which is designed for production and mission critical use cases. It comes with a lot of Enterprise features those are not included with other Open Source databases. Such features are for instance: replicated databases, distributed databases or online backup and recovery.
Ingres is certified with Alfresco 3.3 (and earlier 2.x versions).
Here are links to some tutorials ...