feat: add 'website' field to extension registry.json schema

[jongio]

Summary

Add an optional website field to the extension schema in registry.json so each extension in a multi-extension registry can have its own documentation website URL.

Problem

Currently, registry.json has no website field per extension. In the awesome-azd gallery (PR #749), the extension submission form has a single website field that gets applied to all extensions from a registry. This doesn't work well for multi-extension registries like azd-extensions/registry.json where each extension may have its own docs site.

Proposed Schema Change

Add an optional website field at the extension level:

{
  "extensions": [
    {
      "id": "jongio.azd.app",
      "displayName": "App Extension",
      "description": "...",
      "website": "https://jongio.github.io/azd-app",
      "versions": [...]
    },
    {
      "id": "jongio.azd.copilot",
      "displayName": "Azure Copilot CLI",
      "description": "...",
      "website": "https://jongio.github.io/azd-copilot",
      "versions": [...]
    }
  ]
}

Impact

• azd CLI: Would need to recognize the new field (even if it just ignores it for now)
• awesome-azd gallery: Would read per-extension website and display it on the card, falling back to the issue template value if not present
• Extension authors: Can advertise their docs site directly in the registry

Acceptance Criteria

• Add optional website string field to the extension schema
• Update registry.json schema validation to accept the field
• Update azd extension list or similar to optionally display the website

Related

• awesome-azd PR: feat: integrate azd extensions into awesome-azd gallery awesome-azd#749
• awesome-azd issue: Integrate azd extensions into awesome-azd gallery awesome-azd#748
• Validation migration issue: feat: add 'azd x validate-registry' command for extension registry validation #6896