Adopt Basic Usage chapter for Docker image

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](https://github.com/JanusGraph/janusgraph-docker/discussions/133).

Start with the Getting Started, Installation, and Basic Usage chapters, focusing on the examples that use JanusGraphFactory.open(). Compare their assumptions with the Docker image's JanusGraph Server workflow. Done means the chapter either works for Docker-based usage or clearly identifies its local distribution-archive requirement and avoids confusing users.

Written by the indexing model from the issue text.