Update the API docs.
Update the API docs.

 - Add section headers to build out the table of contents in the generated
   documentation
 - Update the endpoint documentation with new properties for datapoint,
   indicator, and region
 - Add a section to describe the difference between computed and stored
   indicators
 - Add a section to describe how data is aggregated geographically

unicef/polio,unicef/rhizome,SeedScientific/polio,unicef/polio,unicef/polio,unicef/polio,unicef/rhizome,SeedScientific/polio,SeedScientific/polio,unicef/rhizome,SeedScientific/polio,SeedScientific/polio,unicef/rhizome