[contributor] Add sphinxmark to Contributor Guide
[contributor] Add sphinxmark to Contributor Guide

Add sphinxmark to packages that require manual installation
when not using tox.
Add note that the sphinxmark dep Pillow may require
installation of external C libraries on some systems.

Change-Id: I7a001eddeb83a30017fd292cb0cb313b89b2df07

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals