Draft: Identify GitLab Duo features by models

What does this MR do and why?

Technical reference for GitLab Duo Features and allowable models.

Closes Document models used by Duo Features (#452505 - closed)

This table will be extended when additional models are made available by the groupcustom models.

The Model Identifier column is a proposal and can be changed as needed.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.