Add model endpoint identity setup guide

[Paul Lizer (paullizer)]

Summary

• Add a new admin how-to for model endpoint managed identity and service principal setup.
• Cover Azure OpenAI, Foundry (classic), and New Foundry provider choices, RBAC scopes, and modal fields.
• Cross-link the guide from the how-to index, admin configuration, and workspace multi-endpoint documentation.

Validation

• Ran git diff --check for the edited documentation files.
• Checked the new guide for non-ASCII characters.