doc: Add description to new use cases and features

The software stack introduces two new use scenarios by extending the
virtualization stack:

* Inter-VM communication using the shared memory and event channel
  mechanism provided by the Xen hypervisor
* Exposing data with a docker container hosted Nginx

These two use cases are implemented using the OpenAMP framework and
docker container technology.

This patch adds an introduction to these two new use cases and related
features, as well as other related changes.

Signed-off-by: Qi Feng <qi.feng@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Issue-Id: SCM-4877
Issue-Id: SCM-4878
Change-Id: I42c3c98bde9dcd131e662e6cd0ba0b811a17b5e3