docs: Update AlchemyProvider @alchemy/aa-alchemy README

[kevinweaver]

Is this a request for new documentation or updates to existing documentation
Existing Documentation

If existing documentation, a clear summary of what needs to be changed
This README.md documents the creation of an AlchemySmartAccountClient by calling importing and calling the new AlchemyProvider function.

This no longer exists, erroring
Module '"@alchemy/aa-alchemy"' has no exported member 'AlchemyProvider'. on line import { AlchemyProvider } from "@alchemy/aa-alchemy";.

If new documentation, a clear summary of what needs to be added
Document a new example with the newly intended setup code and make it clear what the alternative to AlchemyProvider is.

Additional context
It would also be worth mentioning in the breaking changes post here