Add SignalR Performance Section

**Help us make content visible**

- Search for "aspnet core signalr performance"
- returns link to ASP.NET 4x SignalR: https://docs.microsoft.com/en-us/aspnet/signalr/overview/performance/

**Describe the new topic**

I would like to see similar article (as the link above) in ASP.NET Core, for example under Real-Time Apps: https://docs.microsoft.com/en-us/aspnet/core/signalr/introduction?view=aspnetcore-5.0

*generally speaking - signalr is not as strongly covered for Core vs Framework - both in documentation and on the web, I believe it needs more visibility*