Automate deployment to Play Store and update documentation

[perclasson]

Continuation of #43:

• Automate the deployment to Play store, as it used to be for the old Gallery with fastlane: https://github.com/flutter/flutter/blob/master/dev/bots/deploy_gallery.sh
• Update https://flutter.dev/docs/deployment/cd to include the new steps we took.

[rish07]

Hey @guidezpl, I would like to work on the auto-deployment feature

[guidezpl]

Hey @rish07, thanks for offering, but this would have to be done by a Flutter engineer for permission/access reasons

[rish07]

@guidezpl Thanks for the reply. Can you please take a look at this? I have been trying to get a reply on my approach for a feature request on Flutter Gallery#418

[guidezpl]

I know of someone who can help for that