Add contents in article sidebar for support.rackspace.com

[stephamon]

Steve's team had started working on pulling a contents section from h3 and h4 headers that would display in the article sidebar. This would accommodate our preference for having a contents section for the longer articles. Currently, HTML and Markdown anchor links do not work on support.rackspace.com.

You can see where the Contents section would go here, but it's currently commented out: https://github.com/rackerlabs/nexus-control/blob/master/templates/support.rackspace.com/_includes/article-sidebar.html

I think we'll need to create an HTML file along the lines of this one: https://github.com/rackerlabs/nexus-control/blob/master/templates/support.rackspace.com/_includes/product-nav.html