[RFR] Turn data generator for demo into a public repository

[fzaninotto]

Rationale

• When learning react-admin, developers often need a fake API. jsonplaceholder isn't perfect (it doesn't accept mutations), so developers use JSONServer or json-graphql-server, but they need data.
• The demo doesn't run on CodeSandbox as is because of the dependency to an internal package. Having the demo run in CodeSandbox would ease bug reports
• Who doesn't need fake data?

[fzaninotto]

the name data-generator is already taken

[djhi]

ra-data-generator ?

[fzaninotto]

well, we may publish data generators for blogs, social network, etc in the future. Also, it's in react-admin for now, but honestly nothing forbids that it lives on its own. It's just more convenient to keep it here for now.