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/components/popovers.md

@@ -201,9 +201,9 @@ Options can be passed via data attributes or JavaScript. For data attributes, ap
       <td>number | object</td>
       <td>0</td>
       <td>
-       <p>Delay showing and hiding the popover (ms) - does not apply to manual trigger type</p>
-       <p>If a number is supplied, delay is applied to both hide/show</p>
-       <p>Object structure is: <code>delay: { "show": 500, "hide": 100 }</code></p>
+        <p>Delay showing and hiding the popover (ms) - does not apply to manual trigger type</p>
+        <p>If a number is supplied, delay is applied to both hide/show</p>
+        <p>Object structure is: <code>delay: { "show": 500, "hide": 100 }</code></p>
       </td>
     </tr>
     <tr>
@@ -331,10 +331,10 @@ Toggles the ability for an element's popover to be shown or hidden.
 
 <table class="table table-bordered table-striped table-responsive">
   <thead>
-   <tr>
+    <tr>
       <th style="width: 150px;">Event Type</th>
       <th>Description</th>
-   </tr>
+    </tr>
   </thead>
   <tbody>
     <tr>