doc: tweak doxygen css and remove top border
doc: tweak doxygen css and remove top border

The top border on function names and other doxygen items made
documentation very difficult to read. Remove the top border and make the
auto-generated docs readable.

Signed-off-by: Anas Nashif <0d9952ec84ac43c159f6b7e7ed99a9080c00dd6e@intel.com>

ldts/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,explora26/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr