Make sure installation documentation mentions configuroing the backend configuration

[tm-kn]

https://github.com/wagtail/wagtail-vector-index/blob/main/docs/installation.md

I also wonder if numpy is a good default choice given that it's not production-ready. Maybe we should not allow the user to boot the site until this is configured (Django check framework).

[tomusher]

I don't think it's unreasonable to default to the NumPyBackend so people can get up and running and experimenting quickly, and there's no reason why it can't be production ready, it's just that it might scale poorly in some situations - we might just need to be clearer about that.