Adopt Basic Usage chapter for Docker image

[FlorianHockmann]

The Getting Started part of the docs starts with a chapter Installation where using the Docker image is the preferred method to set up JanusGraph, but in the following Basic Usage chapter the assumption is that JanusGraph is used locally from the distribution archive. It makes heavy use of JanusGraphFactory.open().

We should either rework this chapter to be better compatible with the Docker image where we have JanusGraph Server running and only connect to that, or at least clearly state the fact this chapter only works with the local installation method.

The current situation is confusing to users as can be seen for example in this question.