Add filter functionality

[katydecorah]

filtering the examples list was flagged as an important feature in the mapbox-gl-js docs (see mapbox/mapbox-gl-js#7530 (comment))

refs #65 (comment)

[katydecorah]

@colleenmcginnis as luck would have it, this functionality already exists :) if you pass includeFilterBar={true} to SectionedNavigation the filter will appear at the top!

[colleenmcginnis]

Oh! @katydecorah yes I added that a few weeks ago. My comment wasn't very clear -- if we change the pattern so we don't list all the examples/tutorials in the sidebar then there is nothing to filter. Should we create a new pattern that allows users to filter the cards instead?