Move existing content around based on the doc-migration specification.

Replace :doc: markup with :ref: to have sphinx keep track of where the
files move and generate valid hyperlinks.

Add a few toctrees and index pages for the new directories.

Depends-On: Ia750cb049c0f53a234ea70ce1f2bbbb7a2aa9454
Change-Id: I253ee8f89d3ec40e39310c18bb87ed1d3d5de330
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

In the current doc, the command examples are like
"os server create" but the acutal command should
be started with "openstack" instead of "os", it
is misleading to first time users.

Change-Id: Ie67c0152d8ff8b7c456f91dc8b9a9164437ee9d1

Because option parameter is not required. I add [] to option parameter.

Change-Id: I27c14c42bf576decff90a4a290dcaad7fab00524

Rather than pointing the 'project usage list' entry point directly to
ListUsage, this makes a ListProjectUsage subclass to alias the desired
command and still allow for changing behaviour or issuing a deprecation
messages.  This implementation simply sets a new 'deprecated' attribute
in the command class that cliff will use to skip this command when
generating help output.

The actual skipping of the command in help output is dependant on
https://review.openstack.org/147349 merging in cliff, this review will
simply have no effect on help output until a version of cliff with
that update is present.

The deprecation message printed is somewhat generic until
https://review.openstack.org/147381 is merged in cliff.
Until that is merged, the deprecation message will not have the name
of the actual command given by the user in it, but rather the text
'this command'.

Closes-bug: #1406654
Change-Id: Ie08d4f88d71a660fca1862405351109cd0aa86b6

Change-Id: Iaf911d69a0b63d705f8789a4640018a428b87be6