build: Move doc target

The doc target in the top-level Makefile hides the target of the same
name introduced in rules.mk. This commit moves the rules.mk target to
the documentation Makefile where it cannot leak into the rest of the
build system.

Change-Id: I5f075c59978cc996dec9c48dd4ee40a90b11829a
Signed-off-by: Chris Kay <chris.kay@arm.com>