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.
See the comment for Stable channel.