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

[lesnik512]

Adds a "Shutdown order" subsection to the README, after "Sharing a container".

setup_di appends the root container's close to app.on_cleanup, and aiohttp has no public hook that runs after every cleanup handler, so this is a docs fix. The section lists the hooks that run before the close (on_shutdown, cleanup_ctx teardown, on_cleanup added before setup_di) and says that on_cleanup added after setup_di runs after it. For that case it points to manage_lifespan=False with the existing async with container: example.

I ran the repro from the issue (aiohttp 3.14.5, modern-di 4.1.0). The output matches the issue: only on_cleanup added after setup_di gets ContainerClosedError. With manage_lifespan=False and runner.cleanup() inside async with container:, all six cases resolve a live container.

Closes #52