This commit introduces the following changes:

- The Helm charts page has been repurposed as "Nessie on Kubernetes" guide
  and improved with examples for different databases.
- Portions of the in-tree README for the Helm chart were repurposed (because
  they had little to do with Helm charts):
  - Text related to secrets was moved to the "Nessie on Kubernetes" guide.
  - Text related to minikube was moved to a new "Nessie on Minikube" guide.
- The Docker guide has been slightly augmented, notable to mention the
  different Nessie image repositories and how to pass environment variables.