Rename main storage/datastore/etc. docs since captions now have the name

[dhermes]

We end up just repeating the names. See http://googlecloudplatform.github.io/gcloud-python/latest/

and http://gcloud-python.readthedocs.org/en/latest/

And notice that "Datastore" is both the heading name and the name of the first document in the heading.