"Vault as an External Secret Store" walkthrough improvements

### URL

https://docs.crossplane.io/knowledge-base/integrations/vault-as-secret-store/

### What's Wrong?

- [ ] the sentence `The key d2408335-eb88-4146-927b-8025f405da86 comes from` is incomplete
- [ ] it makes you install a provider, without mentioning that you need to also setup credentials for it, given that it's a walkthrough it would be nice to at least point in the right direction, probably https://marketplace.upbound.io/providers/upbound/provider-gcp/v0.37.0/docs/quickstart.