Refactor Naftiko Specification artifacts

[jlouvel]

@eskenazit Could we refactor the JSON Schema in the repo to align with the spec documentation?

• Rename 'capability-schema.json' file to 'naftiko-schema.json'
• Adjust various artifacts relying on this file name (incl. GitHub Actions)
• Rename "name": "Naftiko Capability Specification" to "name": "Naftiko Specification" in the JSON Schema file itself

Thanks!

[jlouvel]

See related PR: #92