Update user documentation how to run gemini

[smacker]

1. Currently docker-compose builds images which takes forever and fails sometimes. We can use prebuild images instead
2. k8s/ directory and gemini-k8s-cluster are kind of outdated. We can replace them with a link to helm charts. We have charts for standalone gemini and for spark cluster.
3. The default mode is "siva" and if you just follow docker documentation with normal repos gemini wouldn't process anything and it's hard to figure out why