Add Documentation

[korgon]

Base documentation around beacon.js and the various methods/properties available as well as example usage.

• package consumer documentation (how to install, etc...)
• package usage (methods/properties)
• usage on website (in browser) - will need script blocks and example usage (page view, cart view, transaction, etc...)
• headless storefront usage?

This documentation will reside as markdown files in this repository for now.