Add integration quality scale docs (#2457)

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>
Co-authored-by: Franck Nijhof <frenck@frenck.nl>
Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

docs/core/integration-quality-scale/rules/docs-supported-functions.md

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+---
+title: "The documentation describes the supported functionality, including entities, and platforms"
+---
+
+## Reasoning
+
+Users should be able to understand what value the integration will add for their (to be bought) device.
+This will help set users' expectations.
+
+For example, if a user is looking for a new fridge, we should try to be clear about what they can expect from the integration.
+If the integration only supports checking if the door is open or closed, they would be disappointed if they expected to be able to view the temperature of the fridge.
+
+## Example implementation
+
+Example, sorted by entity types:
+
+```markdown showLineNumbers
+## Supported functionality
+
+### Entities
+
+The XY integration provides the following entities.
+
+#### Buttons
+
+- **Start backflush**
+  - **Description**: Starts the backflush process on your machine. You got 15 seconds to turn the paddle after activation.
+  - **Available for machines**: all
+
+#### Numbers
+
+- **Dose**
+  - **Description**: Dosage (in ticks) for each key
+  - **Available for machines**: GS3 AV, Linea Mini.
+  - **Remarks**: GS3 has this multiple times, one for each physical key (1-4), and the entities are disabled by default.
+
+#### Sensors
+
+- **Current coffee temperature**
+  - **Description**: Current temperature of the coffee boiler.
+  - **Available for machines**: all
+  - **Remarks**: When the machine reaches temperature, this will be approximately 3 degrees higher than the `Coffee target temperature`, due to different measurement points.
+
+- **Current steam temperature**
+  - **Description**: Current temperature of the steam boiler.
+  - **Available for machines**: Linea Micra, GS3 AV, GS3 MP.
+  - **Remarks**: -
+
+#### Updates
+
+- **Gateway firmware**
+  - **Description**: Firmware status of the gateway.
+  - **Available for machines**: all
+
+#### Selects
+
+- **Prebrew/-infusion mode**
+  - **Description**: Whether to use prebrew, preinfusion, or neither.
+  - **Options**: Disabled, Prebrew, Preinfusion
+  - **Available for machines**: Linea Micra, Linea Mini, GS3 AV
+
+- **Steam level**
+  - **Description**: The level your steam boiler should run at.
+  - **Options**: 1, 2, 3
+  - **Available for machines**: Linea Micra
+```
+
+Example, sorted by device:
+
+```markdown
+## Supported functionality
+
+### XYZ productname Air Purifier, Air Humidifier and Standing Fan
+
+#### Sensors
+
+- **Filter lifetime remaining**: The remaining life of the filter in number of years. Enabled by default.
+- **Purify volume**: The volume of purified air in cubic meters. Disabled by default.
+
+#### Numbers
+
+- **Favorite level**: Set the favorite level. Possible values are 0 to 10. `0` means it is turned off.)
+- **Volume**: Set the volume. In percent. `0%` means it is off.
+```
+
+## Exceptions
+
+There are no exceptions to this rule.