Improve Sphinx documentation

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`