Add a dedicated getting started section

We see a lot of people having problems to start with JanusGraph, so we have to improve our documentation.

Including:
* Install & Run JanusGraph
- With Docker
- With gremlin-server.sh
- With janusgraph.sh
* Working with JanusGraph
- Adding data
- Query data
- Basic schema management
* Basic Architecture (Storage Backend, Mixed Index Backend and JanusGraph)
* Select a backend
* Next steps (Production, Schema Managment, ...)

Start by reviewing the existing JanusGraph documentation and the issue's listed getting-started topics, including Docker, gremlin-server.sh, and janusgraph.sh. Done means a dedicated section covers installation, basic data and schema work, architecture, backend selection, and next steps.

Written by the indexing model from the issue text.