Break up API Documentation into multiple files

[caleb531]

@Tagl @eliotwrobson I've finished the initial reorganization the API docs, and would love some feedback when you guys have time. I decided to go with the route of adding a docs/ directory with a dedicated file for each section (still in Markdown)

I also decided to add the relevant navigational links to each page for easier browsing, and have updated the headings now that each section has its own file (hooray for less nesting!).

[coveralls]

Coverage remained the same at 100.0% when pulling edea01a97fbad0a4435bb5ddd7acc4aeddaf6ebe on docs-reorg into 0f08e0d04f70bbf12e1adf1030527ebc69684e73 on develop.

[coveralls]

Coverage remained the same at 100.0% when pulling edea01a97fbad0a4435bb5ddd7acc4aeddaf6ebe on docs-reorg into 0f08e0d04f70bbf12e1adf1030527ebc69684e73 on develop.