Demo store for playing with the API

[aldesantis]

Follow-up to #3252.

Stoplight allows users to play with the API directly from the documentation. However, the API host is hardcoded in the documentation, so we would need to set up a Solidus demo somewhere.

There are two challenges I see with this:

• Data integrity issues, although these can be mitigated by resetting the demo every e.g. 24 hours.
• We would need to maintain a different demo for each maintained Solidus version.

With that said, setting up the demos via Heroku and resetting the data periodically should be fairly straightforward, so I'm not sure whether these are actual problems.