(docs) Makefile: re-order targets to build htmldocs-fast by default
(docs) Makefile: re-order targets to build htmldocs-fast by default

rm -rf is not the most user-friendly default action

Signed-off-by: Marc Herbert <77e7c7d746b52c6eb01119b65a031ae1f650246d@intel.com>

nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr