Add example for asp.net core webapplicationfactory to the docs (#1649)

* Add example for asp.net core webapplicationfactory to the docs

* Tweak docs

docs/docs/examples/aspnet.md

@@ -0,0 +1,49 @@
+---
+sidebar_position: 2
+---
+
+# ASP.NET Core Web App/Api
+
+If you want to test a web app, you can utilise the Microsoft.Mvc.Testing packages to wrap your web app within an in-memory test server.
+
+```csharp
+public class WebAppFactory : WebApplicationFactory<Program>, IAsyncInitializer
+{
+    public Task InitializeAsync()
+    {
+        // You can also override certain services here to mock things out
+
+        // Grab a reference to the server
+        // This forces it to initialize.
+        // By doing it within this method, it's thread safe.
+        // And avoids multiple initialisations from different tests if parallelisation is switched on
+        _ = Server;
+
+        return Task.CompletedTask;
+    }
+}
+```
+
+This factory can then be injected into your tests, and whether you have one shared instance, or shared per class/assembly, or a new instance each time, is up to you!
+
+The `IAsyncInitializer.InitializeAsync` method that you can see above will be called before your tests are invoked, so you know all the initialisation has already been done for you.
+
+```csharp
+public class MyTests
+{
+    [ClassDataSource<WebAppFactory>(Shared = SharedType.PerTestSession)]
+    public required WebAppFactory WebAppFactory { get; init; }
+
+    [Test]
+    public async Task Test1()
+    {
+        var client = WebAppFactory.CreateClient();
+
+        var response = await client.GetAsync("/my/endpoint");
+
+        await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.OK);
+    }
+}
+```
+
+Alternatively, you can use the `[NotInParallel]` attribute to avoid parallelism and multi-initialisation. But you'll most likely be sacrificing test speeds if tests can't run in parallel.