The createDecoratorDocs processor identifies docs of exports that actually
represent decorators and get useful information for them.
This commit also set up the rest of the generation to display these new
decorator docs with their own template and updates the API list so that
we can filter the docs by decorator docType.
This allows us to recover the filtering state when navigating back and
forth between the API list and individual API pages.
Closes#490Closes#497Closes#507
Each API index page now displays data from an `api-list.json` file found
locally to that page. This lets us have different API lists for TS, JS
and Dart.
