Add an add-on to document & generate the style guide

[longnd]

What happened

This PR create a new Styleguide add-on for the web project. It will integrate the KSS-node package to document and generate the living style guide.

Insight

• Define a new KSS-node package in package.json
• Define a new script in package.json to generate the style guide

  styleguide.generate": "kss --config .kss-config.json
  

• Generate the configuration file at assets/.kss-config.json
• Edit the endpoint.ex file to allow the webserver to server the priv/static/styleguide directory as the style guide will be generated and stored there.

Proof Of Work

[andyduong1920]

Hi @longnd will we continue on this addon 🤔