Update to use Relay's hook API

[multimeric]

It seems that all the examples in the readme and indeed the example repo use the old createFragmentContainer API, which is no longer recommended: https://relay.dev/blog/2021/03/09/introducing-relay-hooks.

It would be great to have an example (perhaps a new branch in the todomvc repo?) explaining how to do this, because I'm totally lost. In particular the idea of "fragment references" and how to pass them into the top level components is confusing to me.