Documentation reorg: Development docs

Created by: christinaforney

Reference: https://docs.google.com/document/d/1bOGK1s6KeIp8CVbYvb3xOsv6k8_XxQQRLOZVcAGbORc/edit#

This is the first of several PRs, but broke down into steps to make the whole thing easier to understand.

Organized the developer documentation into 3 categories (see above reference for where this structure came from):

1. Getting started
2. How-to guides
3. Background information