Add sample / improve docs when starting with an empty web api project

Related to #202

The "getting started" section explains how to implement a handler. And the samples show how to work with the library in larger projects. But I wasn't able to find any documentation for the following case:

- Create an **empty** web api project
- The generated Program.cs contains

```cs
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

// Remove the following minimal api line
// app.MapGet("/", () => "Hello World!");

app.Run();
```

At least I don't know what is required to register a handler. If you let me know I could work on a PR to improve the docs :)

Start with the getting started section and existing samples, then compare them with the empty project’s generated Program.cs shown in the issue. Document how to register a handler in that setup and ensure the getting-started guidance covers the empty Web API case.

Written by the indexing model from the issue text.