Documentation cleanup (inc. use of <input ... readonly> for static form controls example)

* Fix incorrect code indentation
* Remove unnecessary vendor prefix for `box-sizing` - all modern browsers now support this unprefixed
* Remove incorrect `<label>` and change static controls to readonly inputs
* Allow `<img>` elements without `src` to allow for `holder.js` images used in the docs, which lack `src` and use `data-src` instead

docs/getting-started/introduction.md

@@ -96,9 +96,7 @@ On the rare occasion you need to override it, use something like the following:
 
 {% highlight scss %}
 .selector-for-some-widget {
-  -webkit-box-sizing: content-box;
-     -moz-box-sizing: content-box;
-          box-sizing: content-box;
+  box-sizing: content-box;
 }
 {% endhighlight %}