Add Table of Contents to the API Reference page
Remove Popolo extensions section from reference

The member property on memberships has now been added to the official
Popolo specification, so this is not an extension any more.
Initial pass at membership embed docs
Fix JSON syntax error in embed example
Make sure all urls appear as links

Surrounding a url with angle brackets turns it into a link.
Add docs for merging people with the API

This includes examples of successful and unsuccessful responses.
Update sidebar to include Merging API

Also make some changes to help break the documentation up into slightly
more distinct sections.