Add a dedicated getting started section

[farodin91]

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, ...)

[rngcntr]

Does #1909 meet your expectations? If so, this issue can be closed.

[farodin91]

I think the first 3 parts are done.

I think we should add a next steps section with basic next questions:

• How to select a backend?
• Do I need a index backend?
• How to run JanusGraph in Production?
• How do i load data into JanusGraph for tests?
• How to create a basic Schema for JanusGraph?

[FlorianHockmann]

I added a comment on the PR to restructure the documentation (#2348) and just want to keep it here as it targets the getting started section:

I would like this part to be more or less self contained and ideally explain everything to start a JanusGraph Server instance and play with it a bit. That's why I would move the Gremlin chapter into this part and I think I would also split the JanusGraph Server chapter in the Operations part into two chapters, one that explains how to get started with the pre-packaged distributions and one with the more advanced and optional configuration options. Then we could move the first chapter into the Getting Started area and leave only the second chapter in Operations.