Adding API documentation downloads to the archive

Fixes #933 

Provides download links for the archived API docs. The links link to the zip file, which contains a directory of JSON files. This is currently the only format which we provide the API documentation.

The [dartdoc-viewer](https://github.com/dart-lang/dartdoc-viewer) README shows how these can be viewed and served.

This is staged at https://issue-933-api-downloads-dot-dart-lang.appspot.com/tools/download_archive/

@sethladd @kwalrath @Sfshaza let me know what you think.
@Sfshaza are we using "stable channel" or "Stable channel" or "Stable Channel"?

Also, the stable channel, strictly speaking, doesn't include historical downloads. This page gives you links to historical downloads of stable channel versions. So maybe (assuming you put the docs in the same table):

You can download archived versions of Stable channel downloads of Dart tools and API reference documentation.
To view the API documentation, unzip the archive and then view the JSON files using [dartdoc-viewer].