docs: say which shutdown hooks run after the root container closes

[lesnik512]

Adds a "Shutdown order" subsection to the README, at the end of Usage.

setup_di closes the root container from a provider lifespan it adds to the server, and FastMCP has no public hook that runs after the server lifespan, so this is a docs fix. The section says that the teardown of FastMCP(lifespan=...) and of providers added before setup_di runs after the close. It shows manage_lifespan=False with async with container: in the user's own server lifespan.

I ran the repro from the issue (fastmcp 4.1.0, modern-di 4.1.0). The output matches the issue: the server lifespan teardown and a provider added before setup_di get ContainerClosedError, and a provider added after setup_di resolves fine. With the documented pattern, both failing cases resolve a live container. The new code block also runs together with the Usage example, and tools still resolve through FromDI.

Closes #3