doc: include .svg using the image directive, not raw html

I have no idea what I was thinking when I added them as raw html. Using
the image directive should allow browsers to handle the page layout
better.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>