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 <david.b.kinder@intel.com>