Design the approach for the sidebar in the new blog

[mansona]

This is a follow on from ember-learn/ember-styleguide#209

Instead of implementing a generic sidebar component in the styleguide in one step, we should design a specific sidebar that will work for one project, and then extract the documentation and any useful components into the styleguide so that we can then move on to implement the various other sidebars in other projects.

It is possible that we might end up with a single sidebar that can cover all aspects necessary for all applications, but we shouldn't set out to try and do this as a first step 👍