Add "Getting Started" section to docs

[hCraker]

Describe the feature you'd like added to this project
Currently our docs page doesn't have a "quick start" section like most other PyData packages. Adding a few pages describing why geocat-comp exists, what it is, basic install instructions, and an overview of some of the content would aid new users in getting introduced to the package.

Some potential topics to cover:

• what is geocat-comp and why was it made
• install instructions
• overview of project structure
• talk about what -f2py is and how it works with -comp
• provide examples of basic workflows
• how to get support / report bugs / open GItHub issues

Additional context
Xarray has a really great "Getting Started" page, and we can use it as a model to build off of.

This work is prep for the tutorial in October #239