chore: Update .repo-metadata.json (#5)

Author: averikitsch

.repo-metadata.json

@@ -2,6 +2,7 @@
     "name": "langchain-google-spanner",
     "name_pretty": "Cloud Spanner for LangChain - Python",
     "issue_tracker": "https://github.com/googleapis/langchain-google-spanner-python/issues",
+    "client_documentation": "https://github.com/googleapis/langchain-google-spanner-python/tree/main/docs",
     "release_level": "preview",
     "language": "python",
     "library_type": "OTHER",