css/html introduce proper cell structure

[chrisparnin]

docable is incorrectly adding a cell class to the pre block. This is not the responsibility of docable, nor is it the correct structure we need.

Rather than,

<pre class=docable cell>
    <div class=sideannotation>
    <code>
</pre>

Instead we something like this.

<div class=cell>
    <div class=sideannotation>
    <pre class=docable>
       <code>
    </pre>
	<div class=output></div>
</div>

[chrisparnin]

Fixed.