Add schema reference page for version 2025-06-18

Follow-up to modelcontextprotocol#921.

This adds a schema reference page for version 2025-06-18 of the
specification, comparable to the page that was added for the draft
version.

Author: jonathanhefner

docs/docs.json

@@ -109,7 +109,8 @@
                     ]
                   }
                 ]
-              }
+              },
+              "specification/draft/schema"
             ]
           },
           {