Start ext API documentation for summary visualization about codes

[michaelbrooks]

Select one of the 4-5 different visualizations that will appear on the page for a code (page url will be something like /codes/{id}). Create a draft of the API documentation in docs/ext following the format of the example.

Metadata like basic participant or code names will be retrieved via the REST api, so the visualization's main data series should come from a single URL.

Keep in mind any necessary filtering, sorting, or binning parameters, but also keep these to the bare minimum.

There may be additional examples of documentation currently in the dashboard branch.

[michaelbrooks]

Finished in pulls #65 and #70