fix: correct FirestoreAdminClient source link in documentation

The FirestoreAdminClient source code resides in the `google-cloud-firestore-admin`
directory, which differs from the artifact ID (`google-cloud-firestore`). This
mismatch causes the generated GitHub source links in the documentation to be broken.

This change adds `library_path_overrides` to `.repo-metadata.json` to explicitly
map `FirestoreAdminClient` to the correct source directory.

Fixes: b/442875200

Author: jinseopkim0

.repo-metadata.json

@@ -10,11 +10,14 @@
   "repo": "googleapis/java-firestore",
   "repo_short": "java-firestore",
   "distribution_name": "com.google.cloud:google-cloud-firestore",
+  "library_path_overrides": {
+    "FirestoreAdminClient": "google-cloud-firestore-admin"
+  },
   "api_id": "firestore.googleapis.com",
   "library_type": "GAPIC_COMBO",
   "requires_billing": true,
   "codeowner_team": "@googleapis/api-firestore @googleapis/firestore-dpe",
   "excluded_poms": "google-cloud-firestore,google-cloud-firestore-bom",
   "issue_tracker": "https://issuetracker.google.com/savedsearches/5337669",
   "recommended_package": "com.google.cloud.firestore"
-}
+}