[Website] Provide docker one-liner from Query API pages to try out the queries on that page

[myronmarston]

Suggest an improvement

We want to make it easy for site visitors to try out ElasticGraph to see the value proposition. The Query API is a big part of that, and it'd be good for the Query API pages to highlight a one-liner that can be run in a terminal to boot ElasticGraph locally with the artists schema and fake data.

Note: we want the one-liner to be usable for folks who don't have a Ruby development environment on their machine. It should probably use docker.

URLs of impacted pages

https://block.github.io/elasticgraph/query-api/ and all subpages.

Also this would replace the "Try ElasticGraph Now" call to action at the home page:

https://block.github.io/elasticgraph/

[myronmarston]

My initial (failed) attempt at this is here: #197.