More grid updates

- Flexbox responsive behavior fixed with specific .col-{breakpoint} classes now added
- Dropped the make-col mixin in favor of a column-basics placeholder that we can extend across our grid infrastructure
- Updated docs to use required .col-xs-12 (as a safeguard for when folks enable flexbox mode--this isn't necessary in default grid mode)
- Update flexbox grid docs to include responsive docs, tweak some other bits too

docs/_layouts/docs.html

@@ -21,10 +21,10 @@
 
     <div class="container">
       <div class="row">
-        <div class="col-md-3 push-md-9 bd-sidebar">
+        <div class="col-xs-12 col-md-3 push-md-9 bd-sidebar">
           {% include nav-docs.html %}
         </div>
-        <div class="col-md-9 pull-md-3 bd-content">
+        <div class="col-xs-12 col-md-9 pull-md-3 bd-content">
           <h1 class="bd-title" id="content">{{ page.title }}</h1>
           {{ content }}
         </div>