Add CertificateAuthentication documentation

[JamesNK]

Context: dotnet/aspnetcore#9756 // @HaoK @blowdart

• Explain why this topic is needed.

New authentication mechanism added in 3.0 preview 6.

• Suggest a location in the Table of Contents.

https://docs.microsoft.com/en-us/aspnet/core/security/authentication

• Write an abstract. In one short paragraph, describe what this topic will cover.

How to setup an ASP.NET Core app to use client certificates with authentication using the new APIs.