Move Blazor Circuit Handler content

[rynowak]

We need to create docs for CircuitHandler. This is a server-side blazor feature that lets users register a lifecycle filter for a server-side blazor hive of components

https://github.com/aspnet/AspNetCore/blob/master/src/Components/Server/src/Circuits/CircuitHandler.cs

[nagytam]

An example project to demonstrate the CircuitHandler is available at GitHub: https://github.com/nagytam/BlazorCircuitHandler.

[mkArtakMSFT]

This has been done by @pranavkm: https://docs.microsoft.com/en-us/aspnet/core/blazor/handle-errors?view=aspnetcore-3.1#blazor-server-circuit-handlers

@guardrex let's move this out from the Error handling section. It should be some root-level item under hosting model. Also, this can go into (as a link) under the advanced topics for Blazor.

[guardrex]

Sure thing .. an open PR does a lot of other work to the Hosting Models topic, so I'll take care of this right after that PR merges.