docs: add App specification

[dido18]

Motivation

closes #364
The app-specification.md is the single source of truth of the app.

Change description

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.

[lucarin91]

Can you run task fmt

[CLAassistant]

All committers have signed the CLA.

[dido18]

I would remove the numbers from the headline.

As a general structure, I would adhere to other specifications:

• https://docs.arduino.cc/arduino-cli/sketch-specification
• https://docs.arduino.cc/arduino-cli/library-specification/

[dido18]

how to define a secret variable ? we could give example

[dido18]

variables:
  - name: API_KEY
    description: API Key 
    secret: true

[mirkoCrobu]

Isn't this struct just for the brick-list?
and in the app.yaml is:
arduino-app-cli/internal/orchestrator/app/testdata/complex-app.yaml at main · arduino/arduino-app-cli

[mirkoCrobu]

what do you think?
looking at the code it seems works like this:
Some Bricks define certain variables as secrets. This is declared in the Brick's own definition, not in the app.yaml.
In the app.yaml, the user sets the value of a secret variable exactly like any other variable, using the standard variables map.
Automatic Redaction: When an App is exported, the values of variables marked as secret in the Brick definition (within app.yaml) are automatically redacted (set to empty). This ensures that the App bundle can be safely shared without leaking credentials or private keys.

[dido18]

this is not true, secret is a boolen field

Suggested change

	variables:
	secret: "secret.db_password"
	ports: "8080"
	variables:
	secret: true

[mirkoCrobu]

from tests:
[arduino-app-cli/internal/orchestrator/app/testdata/complex-app.yaml at main · arduino/arduino-app-cli](https://github.com/arduino/arduino-app-cli/blob/main/internal/orchestrator/app