Add tankerkoenig integration (home-assistant#11137)

* Add tankerkoenig integration

* Add new configuration variable "stations"

Enhance the documentation for the new optional configuration variable "stations"

* Fix typos in tankerkoenig integration

* Change integration to platform

The integration is a platform integration instead of just a sensor now.

source/_integrations/tankerkoenig.markdown

@@ -0,0 +1,94 @@
+---
+title: "Tankerkoenig Sensor"
+description: "Instructions on how to integrate Tankerkoenig sensors within Home Assistant."
+logo: tankerkoenig.png
+ha_category:
+  - Energy
+  - Sensor
+ha_release: 0.102
+ha_iot_class: Cloud Polling
+---
+
+The `tankerkoenig` platform allows you to monitor the fuel prices with [tankerkoenig.de](https://www.tankerkoenig.de/) from within Home Assistant and setup automations based on the information.
+One sensor entity will be created for each fuel station within the given radius and for each configured fuel type in it.
+
+You can also add additional stations manually, referencing them via their IDs. To find out the ID for a given fuel station, you can use the [TankstellenFinder](https://creativecommons.tankerkoenig.de/TankstellenFinder/index.html) tool.
+
+## Setup
+
+To use this sensor you need an API key from [tankerkoenig](https://creativecommons.tankerkoenig.de). Go to [api-key](https://creativecommons.tankerkoenig.de/api-key), fill out the form and request a key. The API is free, but requests should be limited to less than once every 5 minutes.
+
+## Configuration
+
+To enable this platform, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+tankerkoenig:
+  api_key: YOUR_API_KEY
+  radius: 1
+  fuel_type:
+    - "diesel"
+```
+
+{% configuration %}
+api_key:
+  description: The api key you got when you registered.
+  required: true
+  type: string
+fuel_types:
+  description: The types of fuels you want to track. Allowed values are `e5`, `e10` and `diesel`.
+  required: false
+  default: ["e5", "e10", "diesel"]
+  type: list
+latitude:
+  description: The latitude of the gas station to list.
+  required: inclusive
+  type: float
+  default: latitude of your home zone
+longitude:
+  description: The longitude of the gas station to list.
+  required: inclusive
+  type: float
+  default: longitude of your home zone
+radius:
+  description: The radius in km. in which to search for gas stations. Cannot be less than 1.
+  required: false
+  default: 5
+  type: integer
+scan_interval:
+  description: The time interval to poll the server for new data. You should not put values lower than 5 minutes here; otherwise you risk your API key being blocked.
+  required: false
+  default: 0:30
+  type: time
+stations:
+  description: List of additional fuel stations to create entities for.
+  required: false
+  type: list
+{% endconfiguration %}
+
+## Full example
+
+This is a full example of the platform:
+
+```yaml
+tankerkoenig:
+  api_key: YOUR_API_KEY
+  fuel_types:
+    - "diesel"
+    - "e10"
+  latitude: 52.51627
+  longitude: 13.3777
+  radius: 1
+  scan_interval: "0:10:01"
+  stations:
+    - 8531b393-1e42-423b-cb4d-e4b98cff8a0c
+```
+
+Assuming there are two fuel stations within the specified range and location, you would get six sensor entities:
+ * sensor.tankerkoenig_berlin_paulstrasse_20_diesel
+ * sensor.tankerkoenig_berlin_paulstrasse_20_e10
+ * sensor.tankerkoenig_aral_tankstelle_diesel
+ * sensor.tankerkoenig_aral_tankstelle_e10
+ * sensor.tankerkoenig_svg_hamburg_strassen>_diesel
+ * sensor.tankerkoenig_svg_hamburg_strassen_e10