Start to blow out and document more flexbox utilities

- Adds new flexbox.md file to utilities docs

- Adds a `breakpoints.yml` data file for easier output of responsive classes in the docs. Will put this to use on other pages soon.

- Adds hella flex utils. There are some dupes for now, but they'll get removed in time.

docs/_data/breakpoints.yml

@@ -0,0 +1,29 @@
+- breakpoint: xs
+  abbr: ""
+  name: Extra small
+  min-width: 0px
+  container: ""
+
+- breakpoint: sm
+  abbr: -sm
+  name: Small
+  min-width: 576px
+  container: 540px
+
+- breakpoint: md
+  abbr: -md
+  name: Medium
+  min-width: 768px
+  container: 720px
+
+- breakpoint: lg
+  abbr: -lg
+  name: Large
+  min-width: 992px
+  container: 960px
+
+- breakpoint: xl
+  abbr: -xl
+  name: Extra large
+  min-width: 1200px
+  container: 1140px

docs/_data/nav.yml

@@ -56,6 +56,7 @@
     - title: Clearfix
     - title: Close icon
     - title: Colors
+    - title: Flexbox
     - title: Display property
     - title: Image replacement
     - title: Invisible content