docs: add App specification

docs/app-specification.md

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+# Arduino App specification
+This is the specification for the `Arduino App` (from now on called App) format to be used with `arduino-app-cli` and `Arduino App Lab`.
+
+# Arduino App Folder structure
+An App is a self-contained folder that includes the following components:
+ - `app.yaml` (mandatory) the file descriptor of the app in YAML format.
+ - `sketch` (optional) the folder containing an Arduino [Sketch](https://arduino.github.io/arduino-cli/1.3/sketch-specification/)).
+ - `python` (optional) the folder containing the Python code.
+
+At least one on `sketch` or `python` folder must be present.
+The App must be self-contained (it does not contain references to external files) because this means it can be exported, shared, or zipped easily.
+
+The user-defined apps are saved into `/home/arduino/ArduinoApps` folder.
+The builtin-apps are stored into `home/arduino/.local/share/arduino-app-cli/examples` folder.
+
+
+Example of a `my-app` folder structure
+```
+my-app/
+    README.md
+    app.yaml
+    sketch/
+        sketch.ino
+        sketch.yaml
+    python/
+        main.py
+```
+
+## `README.md` file
+An (optional) readme file in markdown.
+The link to local resources must be in the same folder of the app. For example, a png inside the folder `myapp/docs/my-banner.png` can be referenced using ![My App](docs/my-banner.png) syntax.
+
+### `app.yaml` file descriptor
+The `app.yaml`  (or `app.yml`) is a YAML file that describes an App.
+
+- `name`: (optional) a short name of the app.
+- `description`: (optional) a brief description of the app.
+- `icon`:  (optional) the emoji of the app
+- `ports`:  (optional) a list of ports to be exposed externally. If not given a random port is opened (if necessary).
+- `bricks` (optional) a list of bricks used by the app with its variable definitions.
+
+Example:
+```yaml
+name: My Arduino App
+description: An example app showcasing what you can do
+icon: 🍓
+ports:
+ - 7000
+
+bricks:
+  - arduino/dbstorage:
+      variables:
+        ROOT_PASSWORD: ${secret.db_password}
+        PORT: 8080
+  - arduino/text-generation:
+      model: gemma-1
+  - arduino/objectdetection:
+      model: yolo
+```
+
+
+### `sketch` sub folder
+The content of the `sketch` subfolder  contains the Ardiuno skecth.
+It must omply with the [Sketch specification](https://arduino.github.io/arduino-cli/1.3/sketch-specification/).
+
+If present it must contain the followign files:
+ - `sketch.ino`
+ - `sketch.yaml` that is compliant to the [Sketch project file](https://arduino.github.io/arduino-cli/1.3/sketch-project-file/)
+
+### `python` sub folder
+The content of the `python` contains the python code.
+
+If present, it must contain the `main.py` with the python code of the main.
+Optionally, a `requirements.txt` with additional python package dependencies to be installed.
+
+### Other
+Other sub-folders or files can be added to the app folder.
+The reserved folder names are `sketch` and `python`.
+The reserved file names are `app.yaml` and `sketch.yaml`.