SignalR Core needs documentation on connection lifecycle

As I get reacquainted with SignalR via SignalR Core, I've found that a lot of the lifecycle operations are no longer supported. It'd be nice to have a page for SignalR Core that outlines some of the connection lifecycle operations as well as some best practices like there is for [ASPNET SignalR](https://docs.microsoft.com/en-us/aspnet/signalr/overview/guide-to-the-api/handling-connection-lifetime-events).

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#### Document Details

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* ID: 22325a88-5e1d-7b54-87b7-ea331a6c73b6
* Version Independent ID: b92a4bf5-f7c9-50b9-d7d7-46ebcf7387b0
* Content: [ASP.NET Core SignalR](https://docs.microsoft.com/en-us/aspnet/core/signalr/?view=aspnetcore-2.1)
* Content Source: [aspnetcore/signalr/index.md](https://github.com/aspnet/Docs/blob/master/aspnetcore/signalr/index.md)
* Product: **aspnet-core**
* GitHub Login: @tdykstra
* Microsoft Alias: **tdykstra**