document new utils and change organization to match css

docs/_data/nav.yml

@@ -60,9 +60,10 @@
     - title: Display property
     - title: Image replacement
     - title: Invisible content
+    - title: Position
     - title: Responsive helpers
     - title: Screenreaders
-    - title: Sizing and positioning
+    - title: Sizing
     - title: Spacing
     - title: Typography
     - title: Vertical align

docs/utilities/position.md

+---
+layout: docs
+title: Position
+group: utilities
+---
+
+Position utilities are helpful for quickly placing a component outside the normal document flow. Choose from a handful of fixed or sticky position classes as needed.
+
+### Fixed top
+
+Position an element at the top of the viewport, from edge to edge. Be sure you understand the ramifications of fixed position in your project; you may need to add aditional CSS.
+
+{% highlight html %}
+<div class="fixed-top">...</div>
+{% endhighlight %}
+
+### Fixed bottom
+
+Position an element at the bottom of the viewport, from edge to edge. Be sure you understand the ramifications of fixed position in your project; you may need to add aditional CSS.
+
+{% highlight html %}
+<div class="fixed-bottom">...</div>
+{% endhighlight %}
+
+### Sticky top
+
+Position an element at the top of the viewport, from edge to edge, but only after you scroll past it. The `.sticky-top` utility uses CSS's `position: sticky`, which isn't fully supported in all browsers.
+
+{% highlight html %}
+<div class="sticky-top">...</div>
+{% endhighlight %}

docs/utilities/sizing.md

 ---
 layout: docs
-title: Sizing and positioning
+title: Sizing
 group: utilities
 ---
 
-## Fixed positioning
+Easily make an element as wide or as tall (relative to its parent) with our width and height utilities. Includes support for `25%`, `50%`, `75%`, and `100%` by default.
 
-The `.pos-f-t` class can be used to easily position elements at the top of the viewport and make them as wide as the viewport. **Be sure you understand the ramifications of fixed-position elements within your project.** Here's how the class is defined:
-
-{% highlight scss %}
-.pos-f-t {
-  position: fixed;
-  top: 0;
-  right: 0;
-  left: 0;
-  z-index: $zindex-navbar-fixed;
-}
-{% endhighlight %}
-
-
-## Width and height
-
-Easily make an element as wide or as tall (relative to its parent) with our width and height utilities. Includes support for `25%`, `50%`, `75%`, and `100%`.
+Width and height utilities are generated from the `$sizes` Sass map in `_variables.scss`. Modify those values as you need to generate different utilities here.
 
 {% example html %}
 <div class="w-25 p-3" style="background-color: #eee;">Width 25%</div>