doc: Makefile: Use phony targets and remove unneeded prereqs

Turn 'content', 'kconfig', and 'prep' into phony targets so that the
docs can be built even if there is a file called e.g. 'kconfig' in
zephyr/doc/.

Also remove the Python scripts as prerequisites from the rules and
reference them directly in the recipes instead. The phony targets are
always "out-of-date", and there are no rules to run for the Python
scripts themselves.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>