Add JSON Schema for documentation and metadata

[simon04]

Relates to #878

[jmerle]

I am not too familiar with JSON schemas, but is this something that should be publicly accessible (i.e. on the devdocs.io domain)? If not, I think this may fit better in the docs/ directory.

[simon04]

is this something that should be publicly accessible

I'd say yes. This allows referencing the schema in JSON files. Here's a big collection of publicly accessible JSON schema definitions, https://www.schemastore.org/json/.