New API docs

This moves away from biscotto and to generation of API doc json data.

Metadata JSON is generated by [donna][donna], and then digested into something for easy html generation by [tello][tello]. The resulting JSON is committed here. I am not sure what exactly we want to do (likely just commit the tello.json).

This cannot be merged until we are ready to move away from the biscotto docs. This work has been started in https://github.com/github/atom.io/issues/638

### The Plan

I want to get the current API doc content out with a new docs site on atom.io. This should make the API changes much easier, as we can add things to the new docs site along with the API docs (eg. marking things essential + extended can all show up as public for a time). Here's how I think we should do it

### TODO

* [x] Convert all comments in core + packages to [atomdoc] in this PR
  * [x] Atom
  * [x] BufferedNodeProcess
  * [x] BufferedProcess
  * [x] Clipboard
  * [x] Config
  * [x] ContextMenuManager
  * [x] Cursor
  * [x] Decoration
  * [x] DeserializerManager
  * [x] Directory
  * [x] Editor
  * [x] EditorView
  * [x] File
  * [x] Git
  * [x] Grammar
  * [x] GrammarRegistry
  * [x] KeymapManager
  * [x] Marker @benogle in text-buffer bo-atom-doc branch
  * [x] MenuManager
  * [x] PackageManager
  * [x] Pane
  * [x] PaneView
  * [x] Point
  * [x] Project
  * [x] Range
  * [x] ScrollView
  * [x] SelectListView
  * [x] Selection
  * [x] Syntax
  * [x] Task
  * [x] TextBuffer
  * [x] ThemeManager
  * [x] View
  * [x] Workspace
  * [x] WorkspaceView
* [x] Add generated `api.json` as a release asset @kevinsawicki
* [x] All external packages need newest grunt-atomdoc
* [ ] Generate docs from `api.json` on atom.io
* [x] Visual Changes
  * [x] Inline args
  * [x] :: instead of @, . for a class method, and gray it out maybe?
  * [x] move examples to be part of the description
  * [x] methods can have examples
  * [x] jump to sections
  * [x] Delineate sections a bit more / strengthen difference between categories
  * [x] just display the description, because the description will always contain the
       summary, or remove summary from description
  * [x] args can have children, e.g. callbacks that have arguments of their own
  * [x] The link to the class file should just use an icon

[donna]:https://github.com/atom/donna
[tello]:https://github.com/atom/tello
[atomdoc]:https://github.com/atom/atomdoc