doc: document customization of autowiring

[adriangb]

Autowiring can easily customized (e.g. to continue auto wiring when a default value is encountered) by overriding Dependant.gather_dependencies.

It would be good to show an examples of that.

Some ideas:

• A depth attribute to limit autowiring depth
• Continuing auto wiring for default values

A nice real world example might be the dependency hierarchy App -> httpx.Client -> httpx.BaseTransport. App accepts a client.
It would be nice to be able to directly inject the transport into the client in a test:

with container.bind(Dependant(lambda: MockTransport(handler=...)), BaseTransport):
    app = container.execute_sync(container.solve(App))

assert isinstance(app.client._base_transport, MockTransport)

[adriangb]

This is now captured by DependantProtocol and documentation around JoinedDependant.