Geospatial App: Update Setup section

[jkarnowski]

Add clarity by organizing prereq/setup by:

• Rename "setup" to "Getting Started"
• Include 2 sections: Learning Objectives and Prereqs on this page of the tutorial

Learning Objectives

Change "Break down "build a full geospatial app from scatch" into the skills you learn as you build" to be a list of skills someone will learn by the end of building the app. for example:

• Create a map
• Add map interactions: zoom, pan, resize window
• Plot data on the map
• Be aware of how to import packages
• Customize imported packages
• Enable the chart to interact with data overlays

Prereqs (or Environment Setup)

• Be familiar with basics of React like Components and state. For example, be aware of this and props and how state changes. (included 'this' and 'props' for an example - feel free to change this to make sense for other React concepts that are relevant for this tutorial)
• Get a Mapbox token
• node >= 6
• yarn or npm package manager installed
• clone this tutorial to your local machine
• Open src/demos/building-a-geospatial-app/starting-code/src/app.js and start with the folder: starting-code.