Node-RED revised documentation - PR 1 of 2 - master branch

Wholesale rewrite of Node-RED documentation to cover:

* How iotstack_nodered gets built
* Credentials encryption and password protection
* How to refer to other containers
* GPIO access (mostly unchanged)
* Sharing files/folders via persistent storage (mostly unchanged)
* Removal of bluetooth (it was wrong; BLE works without host mode; this
was solved ages ago and I should have updated the doco then)
* Full discussion of using SSH to enable the `exec` node to call out
of the container. This includes setup instructions that follow normal
SSH usage patterns (in this case, viewing root inside the container
as a client and doing things from root's perspective) and avoids the
need to pass `-f` parameters etc. Also recommends ed25519.
* How to upgrade Node-RED when a new version is released on DockerHub
* How to customise Node-RED.

Adds three supporting graphics to Containers/images.

Adds new volume mapping to nodered service definition to explicitly
support ssh. This does nothing unless and until a user follows the SSH
setup instructions, and won't break any existing installations that use
the older method.

Adds helper script for finding duplicated nodes to scripts directory.

Author: Paraphraser

.templates/nodered/service.yml

@@ -9,6 +9,7 @@ nodered:
     - "1880:1880"
   volumes:
     - ./volumes/nodered/data:/data
+    - ./volumes/nodered/ssh:/root/.ssh
     - /var/run/docker.sock:/var/run/docker.sock
     - /var/run/dbus/system_bus_socket:/var/run/dbus/system_bus_socket
   devices: