Update documentation to emphasize plugins over [platform] convention

[ehmatthes]

There are two ways to install django-simple-deploy for deployment to Fly.io:

$ pip install django-simple-deploy[fly_io]
$ pip install django-simple-deploy dsd-flyio

Currently, Quick Starts and other documentation emphasizes the first. The second is better because it's less susceptible to quoting issues, but more importantly because it's the same usage as with all other plugins. I really want to emphasize the plugin ecosystem, and showing that from the very beginning seems like the right approach.

• Update usage in these pages:
  • index
  • fly qs
  • upsun qs
  • heroku qs
  • README
• Review rest of documentation for this format.
• Merge (no release necessary for docs)

[ehmatthes]

Working notes

• Started this change for the dsd-upsun Quick Start. That page needs a note for an issue with the upsun create command on Windows, and this change lets us avoid having multiple notes on that Quick Start. See dsd-upsun issue 8.