Set up jsduck

* Declare jsduck config and categories index.

* Remove usage of non-standard '@see' (left-over from VisualEditor).
  We only few and they weren't very useful, pointing to obvious
  static members of the same name). We could set up a plugin
  structure like in VisualEditor, but keeping it simple for now.

* Including oojs.js in the index because of references like
  "@mixins OO.EventEmitter"

* Created externals.js to resolve references to upstream
  libraries that we don't have jsduck-compatible files of
  (unlike oojs.js).

Bug: 57039
Change-Id: I83356b6d51644d5cdec34fc74c22dabfddbf8365