Document public API on main website

Add the API reference here: https://cli.emberjs.com/release/api-documentation/

There's probably some infrastructure work to make this happen automatically, but I think we can start by manually documenting the methods available to an addon in `index.js`? I don't think the methods change very often. Here's the list (taken from https://ember-cli.com/api/classes/Addon.html):

- config
- blueprintsPath
- includedCommands
- importTransforms
- serverMiddleware
- testemMiddleware
- postBuild
- preBuild
- outputReady
- buildError
- included
- shouldIncludeChildAddon
- setupPreprocessorRegistry
- preprocessTree
- postprocessTree
- lintTree
- contentFor
- treeFor