Use Just the HM Docs for the project documentation

As the list of documented reusable features grows, we are likely to need more structure than a standard README will provide, especially to include information about how to contribute and how to document different ways of installing the reusable features.

We already have developed a GitHub Pages template for managing just this type of project; it feels like a good fit here.
https://github.com/humanmade/just-the-hm-docs

Examples from other projects:
https://humanmade.github.io/just-the-hm-docs/
https://humanmade.github.io/asset-loader/
https://humanmade.github.io/webpack-helpers/