Adds a summary line for each guide on docs.openstack.org
in order to help users find the appropriate book for
their needs.
Adds ".docs-link-sections p" to combined.css to style
the summary lines.
Changes applied to Juno, Kilo, Liberty, and Mitaka.
Change-Id: Ibcbc3d18981a58184646f55b93e77032e4f6c1c0
fe5dba946c