Update documentation for Unified HMI

Update documentation for Unified HMI

Updated the README to include support for the Distributed Display
Framework (DDFW) and Flutter applications.
Instructions for environment setup and the demo procedure using the
Unified HMI framework are provided.

Bug-AGL: SPEC-5254

Change-Id: I2a0a05d7307ccb9f0c48c90dcbfa1e64d7c6ed72
Signed-off-by: Kenta <murakami.kenta002@jp.panasonic.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/30498
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Marius Vlad <marius.vlad@collabora.com>