Reorganize docs for easier deploys

With the current docs directory setup, I'm making too many mistakes and have to manually address path changes and directory moves on deploy. This makes for a frustrating experience developing locally and shipping releases. With this PR, we're basically back to the same setup from v3—duplicating the dist directory into our docs directory. Not the most ideal, but very straightforward for me as the release manager.

site/docs/4.1/utilities/visibility.md

@@ -0,0 +1,33 @@
+---
+layout: docs
+title: Visibility
+description: Control the visibility, without modifying the display, of elements with visibility utilities.
+group: utilities
+---
+
+Set the `visibility` of elements with our visibility utilities. These utility classes do not modify the `display` value at all and do not affect layout – `.invisible` elements still take up space in the page. Content will be hidden both visually and for assistive technology/screen reader users.
+
+Apply `.visible` or `.invisible` as needed.
+
+{% highlight html %}
+<div class="visible">...</div>
+<div class="invisible">...</div>
+{% endhighlight %}
+
+{% highlight scss %}
+// Class
+.visible {
+  visibility: visible;
+}
+.invisible {
+  visibility: hidden;
+}
+
+// Usage as a mixin
+.element {
+  @include invisible(visible);
+}
+.element {
+  @include invisible(hidden);
+}
+{% endhighlight %}