docs: update py_wheel to show prefix stripping

While talking to a user, they didn't realize py_wheel had a prefix-stripping ability.

With src-based layouts being common, I think it makes sense to update the example to show how to transform it into the desired wheel output. (This same problem would occur for a repo with multiple sub-projects, too).

Prompt:

```
Act as an expert in Sphinx, MyST, rules_python, Starlark, Bazel.

Update the py_wheel how to guide to show how to transform files using a src-layout.

Use the py_wheel.strip_path_prefixes arg to do this.
```

Locate the py_wheel how-to guide and inspect its current example for a src-based layout. Use the documented py_wheel.strip_path_prefixes argument to show the transformation to the desired wheel output; done when the guide clearly demonstrates the resulting layout.

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