StateManager: add practical Examples to README

[holgerd77]

The StateManager README is still a bit barebone and lacks somewhat of a description of the core concepts like checkpointing, maybe some short intro to StateManager customization, also 1-2 additional practical examples.

It would therefore be nice if this gets a bit more love at some point. ❤️ 🙂

This discussion https://github.com/ethereumjs/ethereumjs-monorepo/discussions/2338 e.g. adds with an example for a state-saving (via passing in a Level DB backed Trie instance) state manager usage. I think this would already an example worth to add.

[holgerd77]

After a short look I would say this is still a thing.

(eventual existing code like the example referenced above needs to be adopted a bit due to our breaking updates on the API (August 2023)).