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

docs/_data/breakpoints.json

@@ -0,0 +1,31 @@
+{
+  "mobile": {
+    "id": "mobile",
+    "name": "Mobile",
+    "from": 0,
+    "to": 768
+  },
+  "tablet": {
+    "id": "tablet",
+    "name": "Tablet",
+    "from": 769,
+    "to": 1023
+  },
+  "desktop": {
+    "id": "desktop",
+    "name": "Desktop",
+    "from": 1024,
+    "to": 1215
+  },
+  "widescreen": {
+    "id": "widescreen",
+    "name": "Widescreen",
+    "from": 1216,
+    "to": 1407
+  },
+  "fullhd": {
+    "id": "fullhd",
+    "name": "FullHD",
+    "from": 1408
+  }
+}