docs: add Microsoft OAuth section to Built-in IdP guide

[k1LoW]

This pull request updates the documentation for the Built-in IdP to add support for Microsoft OAuth authentication in addition to the existing Google OAuth integration. The changes clarify configuration requirements, update examples, and ensure that all relevant sections mention Microsoft OAuth where appropriate.

Authentication provider support:

• Added documentation for Microsoft OAuth support, including a new section explaining how to enable "Sign in with Microsoft" via Microsoft Entra ID, configuration requirements, and user creation behavior.
• Updated the authentication options list and configuration examples to include allow_microsoft_oauth and its requirements, ensuring Microsoft OAuth is presented alongside Google OAuth. [1] [2]

Configuration and requirements:

• Updated the disable_password_auth documentation to indicate that either Google OAuth or Microsoft OAuth (or both) can be used as the sole authentication method when password authentication is disabled. [1] [2]
• Clarified configuration requirements and warnings to include Microsoft OAuth, such as the need for allowed_email_domains and the incompatibility with non-email identifiers. [1] [2]

General improvements:

• Updated summary and feature lists to reflect support for both Google and Microsoft OAuth for streamlined sign-in.