doc: add output directory to .gitignore

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ic1634d2682636c99946fe288f09d5977c37e44f1

Reviewed-on: https://review.gerrithub.io/415825


Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>