Glossary should be used to describe common terms used in Manila's
documentation. Starting using it for "manila-share", "manila-api" and
"manila-scheduler".
Change-Id: I288332f5b599c9c78d45f0102e4d2a20753af3a6
Add doc for an Introduction to Manila dealing with the fundamentals and basic
terminology used with respect to Manila from an Admin perspective.
Also, create a directory for an Admin ref along with a sample index.rst.
Change-Id: I1847f291750227927a747b5ea224a6861310a529