docs(image): add Image provider getting-started guide

- Add getting-started page with prerequisites, installation, and usage
- Register Image provider section in docs navigation

docs/docs.json

@@ -255,6 +255,12 @@
                   "user-guide/providers/cloudflare/authentication"
                 ]
               },
+              {
+                "group": "Image",
+                "pages": [
+                  "user-guide/providers/image/getting-started-image"
+                ]
+              },
               {
                 "group": "LLM",
                 "pages": [