Roadmap page for the Overview section

[benjagm]

This is part of the Release 3 of the #158 . It will be great to create a page with the JSON Schema Roadmap.

Published roadmap: https://github.com/orgs/json-schema-org/discussions/427

The idea is create a page similar to this one : https://docs.dapr.io/contributing/roadmap/

But embedding the Roadmap github board : The JSON Schema Roadmap (view)

Other references: https://pantos.io/en/roadmap