docs: Use cached versions of Cloud objects.inv files (#863)

The objects.inv files for two different library deps
are currently inaccessible due to redirects.

In the meantime, allow continuing to build the docs by caching
them locally.

They are second in the list, so if the original URL's
start serving the files themselves, there will be no need
to update conf.py again. The cache is strictly a fallback.

Fixes #862

Author: rwhogg

packages/google-cloud-ndb/docs/cloud-core_objects.inv

@@ -214,16 +214,16 @@
     "python": ("https://docs.python.org/", None),
     "google-auth": ("https://google-auth.readthedocs.io/en/latest/", None),
     "google-cloud-datastore": (
-        "https://googleapis.dev/python/datastore/latest/",
-        None,
+        "https://cloud.google.com/python/docs/reference/datastore/latest/",
+        (None, "datastore_objects.inv"),
     ),
     "google-api-core": (
         "https://googleapis.dev/python/google-api-core/latest",
         None,
     ),
     "google-cloud-core": (
-        "https://googleapis.dev/python/google-cloud-core/latest",
-        None,
+        "https://cloud.google.com/python/docs/reference/google-cloud-core/latest/",
+        (None, "cloud-core_objects.inv"),
     ),
     "grpc": ("https://grpc.io/grpc/python/", None),
 }