doc: disable intersphinx mapping

The intersphinx extensions let one reserve a link namespace and map it
to a remote URL, for example to refer to the Python documentation. A
drawback is that, from time to time, Sphinx will attempt to download an
object inventory from the remote, that is not convenient when running
sphinx in offline mode such as in a plane.

Change-Id: Idef8e88545715a3f1cb9ace002960a7ad3e37b69
Merge "doc: disable intersphinx mapping"