Sort out structure of documentation

[wtbarnes]

There was some discussion during the hack days and in the chat room about what niche this package would fill. In addition to providing methods, it would be nice if the documentation could serve as a consolidated guide with information about a variety of DEM inversion methods.

This could provide background on the theory (e.g. how the methods are derived) as well as their limitations and a plethora of resources. It'd be even better if this info could be coupled with a tutorial inline on how to use each method.

@PaulJWright has some prepared material on the XRT and regularized DEM methods that would make for a useful start.

[wtbarnes]

So I think we would want at least the following pages,

• Getting Started (install instructions, intro to the package, etc.)
• User Guide
  • How to write your own model
  • Existing Models (theory plus tutorial of models included in the package)
    • XRT
    • HK12
    • etc.
• Resources (lots of links basically; could start off very not curated and we could iterate on it)

As we don't have any model implementations yet, we could start off by just writing some theory/background for a few models and then including code once we have the implementations in.