chore: adding docfx doclet resource (#362)

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Author: yoshi-automation

.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

@@ -4,14 +4,14 @@
  "git": {
  "name": ".",
  "remote": "https://github.com/googleapis/java-core.git",
- "sha": "1890399290149b8ccbe83f759513e55837742049"
+ "sha": "0f2eadd32020b546e71332bf4009e4c9195ee72a"
  }
  },
  {
  "git": {
  "name": "synthtool",
  "remote": "https://github.com/googleapis/synthtool.git",
- "sha": "6133907dbb3ddab204a17a15d5c53ec0aae9b033"
+ "sha": "3816b080296d4d52975079fd26c110dd26ba25af"
  }
  }
  ],