Update README.md

Fix link to demo in README.md (dev)

Update _variables.scss

Update navbar.html

Update _config.yml

Update docs.yml

Fix font, 500 not exists

Update README.md

Update index.md

Update Gemfile

Fix fo Windows

Add `Setup environment`, `Build locally` sections.
Move up `Status` section.

Update installation guide

Update _variables.scss

Update alerts.md

Update demo.js

FF fix