[Documentation] Explain how to sign-in users in a Web App with multiple IdPs (Azure AD, Azure ADB2C, and OAuth 2.0 providers)

[jmprieur]

Documentation Related To Component:

Sign-in in Web app:
Explain how to sign-in users in a Web App with multiple IdPs (Azure AD, Azure ADB2C, and OAuth 2.0 providers)

Please check those that apply

• typo
• [x ] documentation doesn't exist
• documentation needs clarification
• error(s) in example
• needs example

Description Of The Issue

See ASP.NET Core issue dotnet/aspnetcore#11972
@alexvy86 created a repo with a sample app that uses 3 different auth providers, based on the official Microsoft samples. It does require some setup with the app registrations in AzureAD and AzureAD B2C, but can serve as a starting point.

See the repo https://github.com/alexvy86/multi-auth-sample/

[jmprieur]

See also #549