[DOC] Clean up AZURE_CLIENT_ID and AZURE_CLIENT_SECRET usage

### Willingness to contribute

Yes. I can contribute for this issue independently.

### URL(s) with the issue

_No response_

### Description of proposal (what needs changing)

In latest feature registry, feathr direct access to registry database is deprecated, as a result, registry service is the only component that can access registry database.

This means the usage of AZURE_CLIENT_ID and AZURE_CLIENT_SECRET should get deprecated and following usages should get clean'ed up

1. Usage in deployment. Deployment should only use managed identity.
2. Usage in sample notebooks. All notebook users should use api_endpoint and point it to the registry endpoint get provisioned by deployment.
3. Usage in document. The only place to mentioned AZURE_CLIENT_ID and AZURE_CLIENT_SECRET is document is direct purview access, and should note this is deprecated soon.