Add dashing config file for generating docset from flutter docs

[imWildCat]

The aim of this PR is to introduce .docset for Flutter APIs in relation to #9955.

A brief description about building this docset:

Prerequisites

1. A clone of the Flutter repository
2. Installed technosophos/dashing

Docset building procedure

1. Run the script dev/bots/docs.sh to build Flutter docs.
2. Execute dashing build --source ./doc --config dashing.json to build the docset.

Issues of this configuration file

1. There is no configuration field for the version number in the requirements of dashing.json. The version number could be specified at a 'Dash documentation feeds' file. For more details, please read: https://kapeli.com/docsets#dashdocsetfeed. In addition, here is my minimal example: https://github.com/imWildCat/flutter-docset/blob/master/flutter.xml.

2. As for icon32x32 in dashing.json, dev/docs/doc/flutter/static-assets/favicon.png is used currently. This icon image works well in the Dash.app though this image is actually '64x64'.

3. The selectors of different entry types should be improved for better doc navigation.

References

1. Supported Entry Types - Docset Generation Guide: https://kapeli.com/docsets#supportedentrytypes
2. technosophos/dashing: A Dash Generator Script for Any HTML: https://github.com/technosophos/dashing

[gspencergoog]

Thanks so much!