Update and correct docs for new features/changes introduced in #988

[unmarshall]

How to categorize this issue?

/area documentation
/kind enhancement

What would you like to be added:
A lot of changes were introduced as part of #988. There were also some misses that needs to be corrected.

• Update mkdocs.yml file and add usage/using-druid-client.md to the navigation.
• Link deployment/prepare-helm-charts.md and also make it accessible via mkdocs rendered docs.
• Correct terminal msg at

  etcd-druid/hack/prepare-chart-resources.sh

  Line 24 in 7bcd5b9

  echo -e "CRD ${crd} not found in ${PROJECT_ROOT}/api/core/crds, run 'make generate' first"

  . Instead of make generate just use make --directory=api generate