Add links to external auth samples

[Tratcher]

AzureAd:
https://github.com/azure-samples?utf8=%E2%9C%93&q=active-directory+core&type=&language=
https://github.com/Azure-Samples/active-directory-dotnet-webapp-webapi-openidconnect-aspnetcore

IdentityServer: https://github.com/IdentityServer/IdentityServer4.Samples

OpenIdDict: https://github.com/openiddict/openiddict-samples