Add npm docs

This is a complete rewrite of the site, and a work in progress. The main goals are:

- Fetch and include [npm's documenation](https://github.com/npm/npm/tree/master/doc) in a repeatable and automate-able fashion
- Allow anyone to update site content by simply editing markdown files.
- Allow authors to add metadata to pages, like title, keywords, date, custom layout, etc.
- Allow markdown files to be renamed or added without additional configuration.

The new README provides a high-level description of how the app currently works: https://github.com/npm/docs.npmjs.com/tree/add-npm-docs#readme