[DocumentIntelligence] Update README to highlight identity-based authorization

[kinelski]

This issue applies both to the Form Recognizer and to the Document Intelligence packages.

Currently we start by introducing API key authentication and later present identity-based authorization as a secondary authentication alternative. We should:

• Invert the way these are presented, highlighting identity-based authorization.
• Update the terminology used, replacing "Azure Active Directory" with new vocabulary.

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @ctstone @vkurpad.