Should be relatively simple - we just need to be able to make a request to the bundled server for a given article and be returned a JSON response containing an array of section data. We need the HTML for each section, but probably also the section title info.

Note this changes nothing about the bundled server's handling of index.html - those changes are detailed here: T175322

Initial thoughts here:
https://phabricator.wikimedia.org/T174711#3587290