hubcontext.md: updates examples (#18284)

* Update hubcontext.md

Failing to call next causes bad things to happen. Better to leave it in the example.

* Update hubcontext.md

This shows how to access IHost for use outside of aspnetcore. ie integration with other dependency injection frameworks

* hubcontext.md: Move note back to original location.

* Update hubcontext.md

* Update aspnetcore/signalr/hubcontext.md

Co-authored-by: Brennan <brecon@microsoft.com>

Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>
Co-authored-by: Brennan <brecon@microsoft.com>

Author: 3 people

aspnetcore/signalr/hubcontext.md

@@ -44,12 +44,40 @@ app.Use(async (context, next) =>
     var hubContext = context.RequestServices
                             .GetRequiredService<IHubContext<MyHub>>();
     //...
+    
+    if (next != null)
+    {
+        await next.Invoke();
+    }
 });
 ```
 
 > [!NOTE]
 > When hub methods are called from outside of the `Hub` class, there's no caller associated with the invocation. Therefore, there's no access to the `ConnectionId`, `Caller`, and `Others` properties.
 
+### Get an instance of IHubContext from IHost
+
+Accessing an `IHubContext` from the web host is useful for
+integrating with areas outside of ASP.NET Core, for example, using 3rd party dependency injection frameworks:
+
+```csharp
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            var host = CreateHostBuilder(args).Build();
+            var hubContext = host.Services.GetService(typeof(IHubContext<MyHub>));
+            host.Run();
+        }
+
+        public static IHostBuilder CreateHostBuilder(string[] args) =>
+            Host.CreateDefaultBuilder(args)
+                .ConfigureWebHostDefaults(webBuilder => {
+                    webBuilder.UseStartup<Startup>();
+                });
+    }
+```
+
 ### Inject a strongly-typed HubContext
 
 To inject a strongly-typed HubContext, ensure your Hub inherits from `Hub<T>`. Inject it using the `IHubContext<THub, T>` interface rather than `IHubContext<THub>`.