doc: Update developer manual and user guide

The main changes are as below:

* Add introduction to device tree nodes for shared memory and event
  channel in Xen.
* Add the description to the configurations for Linux kernel to run as
  Xen VM, enable shared memory, run docker, etc.
* Introduction to the demo applications.
* Bump Zephyr version to 3.2.0.
* Update the introduction to Zephyr sample applications

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: I12dfa0d38062119ab7e306adf78b8b81c9a96250