Update responsiveness breakpoints values in docs (#1264)

Update responsiveness breakpoints values in docs (#1264)

* 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