doc: add doc home link in left nav
doc: add doc home link in left nav

It's been annoying that there was no convenient way to get back to the
documentation home page.  There is a link in the breadcrumb trail at the
top of the page, but folks didn't notice that.  This PR adds a
"Documentation Home" link at the top of the left-nav menu.

Signed-off-by: David B. Kinder <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

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