chore: adding docfx doclet resource (#511)

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Source-Link: googleapis/synthtool@3816b08

.kokoro/release/publish_javadoc.cfg

@@ -27,3 +27,6 @@ before_action {
     }
   }
 }
+
+# Downloads docfx doclet resource. This will be in ${KOKORO_GFILE_DIR}/<doclet name>
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/docfx"

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-firestore.git",
-        "sha": "465d4a1969f6df2b139c707967fb0adad44e18bd"
+        "sha": "18879321123e74dc6763cb355a4d14a7d2f18b20"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "6133907dbb3ddab204a17a15d5c53ec0aae9b033"
+        "sha": "3816b080296d4d52975079fd26c110dd26ba25af"
       }
     }
   ],