[DESIGN] Dashboard create command [IOT-1388] (#40)

* Draft dashboard create solutions

* Improve commands readability

* Adapt to new docs template

* Mark "multiple things support" as accepted solution

Author: Paolo Calao

designs/0001-dashboard-create.md

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+# Dashboard create command
+
+## Review Period
+
+Best before October, 5, 2021.
+
+## Overview
+Arduino Cloud CLI should allow users to create a dashboard starting from a cloud template.
+
+The following User story has been taken from the [RFC](https://arduino.atlassian.net/wiki/spaces/FTWEB/pages/2761064740/Arduino+Cloud+CLI).
+* User is able to launch a simple CLI command to create a Dashboard in Cloud
+  * the new Dashboard to create is a logical copy of another existing “template” Dashboard
+  * the newly created Dashboard is displaying variables coming from a Thing specified during creation
+
+## Problem
+**An implementation for this command should be chosen.**
+
+The RFC specifies that this command is going to work only for dashboards with a single thing. However, dashboards with multiple things are common and **it would be better to introduce this feature from the beginning.**
+
+The problem in supporting multiple things for a single dashboard lays in mapping the things described in the dashboard template to the actual things.
+Indeed, if a single thing is used, it can be passed as a simple flag into the command.
+
+## Accepted solution
+
+It has been decided to implement the **Multiple things support** solution because it was the most future-proof.
+### Multiple things support
+
+Let's take a dashboard template:
+```YAML
+id: home-security-alarm-dashboard
+name: Home Security Alarm
+widgets:
+  - type: Messenger
+    name: message_update
+    variables:
+      - thing_id: home-security-messenger
+        variable_id: message_update
+  - type: Switch
+    name: light_alarm
+    variables:
+      - thing_id: home-security-alarm
+        variable_id: light_alarm
+```
+Here we are going to use a flag, that could be called `override`, which takes as parameter a map to be defined with the following structure: `<thing-placeholder>=<thing-id>,..`
+
+Following the example, the command will be something like:
+
+```sh
+arduino-cloud-cli dashboard create --name <dashname> --template <yamldashtemplfile> --thing-override home-security-alarm=<mythingid1>,home-security-messenger=<mythingid2>
+```
+
+
+## Alternative solutions
+### Single thing support
+
+This is the simplest solution but it's very limiting.
+
+Let's take a simple example:
+```YAML
+id: home-security-alarm-dashboard
+name: Home Security Alarm
+widgets:
+  - type: Messenger
+    name: message_update
+    variables:
+      - thing_id: home-security-alarm
+        variable_id: message_update
+```
+In this example, the dashboard uses a single thing. So the command could easily be something like: 
+
+```sh
+arduino-cloud-cli dashboard create --name <dashname> --template <yamldashtemplfile> --thing-id <mythingid>
+```
+
+or even:
+
+```sh
+arduino-cloud-cli dashboard create --name <dashname> --template <yamldashtemplfile> --thing-override <home-security-alarm>=<mythingid>
+```