* Add breakpoints variables in responsivness docs page
* Update breakpoints list values by looping through page.breakpoints
* Add link to responsiveness variables repo file
* Add mixins variables in responsivness docs page
* Update mixins list values by looping through page.mixins
* Add link to responsiveness mixins repo file
* Update responsivness demo table header
* Add responsivness variables reference table
* Add variables to repo files urls (dry)
* Add breakpoints as jekyll data
* Use responsiveness breakpoints from jekyll data
* Replace breakpoints.yml with breakpoints.json and update relevant code accordingly
* Update URLs to source files in responsiveness docs page
