Use rjsf.js.org as domain for docs and playground

[jasny]

Javascript projects can get a free subdomain of js.org. I want to suggest using rjsf.js.org for the playground and docs.

The current URL https://rjsf-team.github.io/react-jsonschema-form for project root isn't great.

[heath-freenome]

@jasny I'll check in with the @epicfaace, the project owner, to see if he wants to put the work into doing this.