refactor: update registry type from 'docker-hub' to 'oci' across seed data (#325)

Updates the registry type for all Docker Hub packages in the seed data
from `docker-hub` to `oci` to align with the standardized OCI registry
format. This change affects the `registry_type` field and simplifies the
`identifier` field by removing the full Docker Hub URL prefix.

Changes:
- Updated `registry_type` from `docker-hub` to `oci` for all Docker
packages
- Simplified `identifier` field to use just the repository name/path
instead of full URLs
- Maintained `registry_base_url` as `https://docker.io` for all entries

This refactoring improves consistency and aligns with OCI standards
while maintaining backward compatibility through the registry base URL.

docs/server-json/examples.md

@@ -131,7 +131,7 @@ This will essentially instruct the MCP client to execute `dnx Knapcode.SampleMcp
         ]
       },
       {
-        "registry_type": "docker-hub",
+        "registry_type": "oci",
         "registry_base_url": "https://docker.io",
         "identifier": "mcp/filesystem:1.0.2",
         "version": "1.0.2",
@@ -339,7 +339,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
     },
     "packages": [
       {
-        "registry_type": "docker-hub",
+        "registry_type": "oci",
         "registry_base_url": "https://docker.io",
         "identifier": "example/database-manager-mcp",
         "version": "3.1.0",