SKASubarray.rst

Building on newer Sphinx versions was not working correctly.
E.g., Tango commands, attributes and properties were omitted.
The commands which use decorators are now working.  Device attributes
defined like `myAttr = attribute(..)` are now visible, using a workaround
in the autodoc mock.  Similarly, device properties also use the
workaround.

Removed some duplication, and an unused requirement: `future`.

Improved layout and updated to be more like the SKA theme,
based on ska-mccs-low.  This includes links to the source code.

Added a make target to build locally:  `make docs-in-docker`