docs: updated the SSO docs for microsoft so its more clear for users.

[ltechconsultancy]

This PR updates the Microsoft SSO documentation to enhance clarity for users configuring this authentication method.

Previously, while the documentation mentioned the necessary redirect URI (<open-webui>/oauth/microsoft/callback), it wasn't explicitly stated that this also needed to be set as the MICROSOFT_REDIRECT_URI environment variable. This could lead to confusion during setup.

This update addresses this by:

• Adding MICROSOFT_REDIRECT_URI to the list of required environment variables.
• Explicitly stating that the value for MICROSOFT_REDIRECT_URI must be <open-webui>/oauth/microsoft/callback, aligning it with the redirect URI configured in the Microsoft OAuth application.

These changes aim to make the setup process more straightforward and ensure users correctly configure all necessary environment variables for Microsoft SSO with Open WebUI.

[tjbck]

Thanks!