README improvements for first time users

[aclel]

I went through the README from scratch on a Windows machine to check if the order of the instructions made sense. This PR is proposing to reorder parts of the README so that everything that a first time user needs to get ginan running is in one spot.

• Getting started with Docker has been moved to the top. The AppImage is simple to use, but it does not have any example data so it's not easy for a user to run ginan for the first time.
• mkdir ginan-data has been added in the docker instructions so that a user does not encounter an error that ${host_data_folder} does not exist
• The downloads with getData.sh and getProducts.sh have been moved up to the Getting Started section from the Build section
• Ready! section has been moved to Getting Started
• Python tools instructions has been moved to the Getting Started section instead of Installation from source section
• Mongodb has been disabled in all examples, because it's unlikely that a first time user will have mongodb running, and it's just another option that they're going to have to find and turn off