Readd main-landing.css
Readd main-landing.css

This file is needed for for docs.openstack.org/infra and
specs.openstack.org.

Readd it and add comment about usage.

Change-Id: I979de7c14f336f3fda18ac5e9f4f582e3518cb12

AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals