Add documentation for Atag integration (#11841)

* Add documentation for Atag integration

* ✏️ Tweak

* Update source/_integrations/atag.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/atag.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/atag.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/atag.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/atag.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update atag.markdown

* - updated to reflect simplified configuration
- removed warnings related to setup through configuration.yaml

* Update source/_integrations/atag.markdown

missed this one

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update atag.markdown

* move atag.png to branch repo

* ✏️ Tweaks

Co-authored-by: Klaas Schoute <klaas_schoute@hotmail.com>
Co-authored-by: Franck Nijhof <frenck@frenck.nl>
Co-authored-by: Nelissen <Mats.Nelissen@ing.com>

source/_integrations/atag.markdown

@@ -0,0 +1,77 @@
+---
+title: Atag
+description: Instructions on how to setup Atag integration.
+ha_category:
+  - Climate
+  - Water heater
+  - Sensor
+ha_release: 0.109
+ha_iot_class: Local Polling
+---
+
+The `Atag` integration allows Home Assistant to connect to [Atag One](https://atag-one.com) thermostats, reporting and setting its status.
+The integration implements the following platforms:
+
+- Climate
+- Water Heater
+- Sensor
+
+## Configuration
+
+The Atag integration can be enabled directly from Home Assistant. Navigate to `configuration`, then `integrations` and click `add`. Click `Atag` to initiate the configuration. Only the IP Address has to be provided, but be sure to add your email address if you experience connectivity issues.
+
+{% configuration %}
+host:
+  description: Atag hostname or IP address.
+  required: true
+  type: string
+email:
+  description: Email registered in the Atag App.
+  required: false
+  type: string
+port:
+  description: Port to reach the Atag API. Only needed if connecting through alternative routes.
+  required: false
+  type: integer
+{% endconfiguration %}
+
+## Climate
+
+The `Atag` climate platform provides current and target temperature information for the heating system, boiler status and HVAC mode.
+
+### Integration services
+
+This integration supports the following services (see [Climate](/integrations/climate/)).
+
+- [`set_temperature`](/integrations/climate/#service-climateset_temperature)
+- [`set_hvac_mode`](/integrations/climate/#service-climateset_hvac_mode)
+  - `heat` for regular thermostat mode
+  - `auto` for weather-based mode
+  - `off` to disable control from Home Assistant
+
+<div class='note'>
+HVAC mode provides returns regular or weather-based mode (i.e., not manual, auto, extend, fireplace).
+The hold modes (manual, auto, extend) are currently available as a sensor only. Setting these modes is planned for a future update.
+</div>
+
+## Water Heater
+
+The Water Heater reports current and target temperature for Domestic Hot Water demand, as well as boiler status (heating or idle). This can be used to detect hot water demand, such as a running shower or tap water.
+Setting target values is currently not supported.
+
+## Sensor
+
+Not all metrics reported by the One are part of either the Water-Heater or Climate platform. Additional sensors are added to Home Assistant to monitor these metrics, which can be disabled in the UI if preferred. Navigate to `Configuration`, `Entities` and select the entity you wish to disable.
+The following sensors will be added to Home Assistant:
+
+### Sensors enabled by default
+
+- `outside_temp`
+- `outside_temp_avg`
+- `weather_status`
+- `operation_mode`
+- `ch_water_pressure`
+- `dhw_water_temp`
+- `dhw_water_pres`
+- `burning_hours`
+- `flame_level`