revise JSON-schemas

[lzehl]

The JSON-schemas should be usable with a common JSON-schema validator to validate our JSON-LDs.
NOTE: compact JSON-LD representations seems to be not supported by common JSON-schema validators.

[Raphael-Gazzotti]

Currently, the $ref references in our JSON Schema are not valid JSON Schema references. To resolve this, we need to update them to point to valid schemas instead of linking to the documentation.

This is related to #1, where we can explore the option of using embedded schemas as a potential solution. Another quick fix could be to link them to the Github resources.