Update Developer Documentation for Setting Up Self-hosted Models Environment Locally

Description

There is a gap in documentation for GitLab developers in external teams who are wishing to get started with GitLab Duo Self-Hosted features. While we have some dev docs in the Custom Models repo, these docs are not updated regularly and have plenty of gaps too.

While we do have customer-facing documentation that are regularly updated and are a source of truth, it does not cover nuances of working with the feature from a GitLab developer perspective E.g internal setup scripts, dev ENV variables, workarounds etc.

Potential gaps to cover:

• Active feature flags
• ENV variables and their nuances e.g when to use CLOUD_CONNECTOR_SELF_SIGN_TOKENS=1 and when not to?
• Setup scripts e.g GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup[duo_enterprise] for creating Enterprise environment
• Guidance on getting access to AWS credentials for Bedrock models
• Guidance on getting access to staging-ref for testing
• Guidance on best models for local testing

Definition of done

• Developer documentation for working with GitLab Duo Self-Hosted is up to date and useful for other teams when setting up their development environment