Create custom examples gallery

[jacobtomlinson]

As outlined in rapidsai/cloud-ml-examples#197 we need to have a gallery-style page showing all of the example notebooks with their associated tags. We then need to add a filtering sidebar menu to allow users to find the notebooks they are interested in.

I expect this involves writing a custom directive that walks the toctree discovering the notebooks. Then walking the notebooks and loading the tags from the first cell metadata. Then render out the grid using the same nodes we are building manually on other pages currently.

For the filtering sidebar we need to drop in a custom sidebar object with all of the tag categories and then write some client-side JavaScript to show/hide grid items depending on which filters are checked.