feat: link each platform and system to its data

An emulator page linked its YAML and api/v1 JSON, platform and system
pages nothing. Each platform page now links its YAML, its own
api/v1/platforms/<id>.json and its install manifest, and each system
page its api/v1/systems/<slug>.json, both validated by the envelope
schema.

scripts/validate_schemas.py

@@ -419,7 +419,9 @@ def main() -> int:
             api_dir / name
             for name in ("catalog.json", "platforms.json", "emulators.json", "gaps.json")
             if (api_dir / name).exists()
-        ]
+        ] + sorted((api_dir / "platforms").glob("*.json")) + sorted(
+            (api_dir / "systems").glob("*.json")
+        )
         errors.extend(
             _validate_json_files(envelope_paths, "site-api-envelope.schema.json")
         )