Extract developer site (#5249)

* Extract developer site

* Fix title in sidebar

* Update dev section reference

* Update edit in github link on help page

source/developers/platform_example_sensor.markdown

@@ -9,58 +9,6 @@ sharing: true
 footer: true
 ---
 
-This is a minimum implementation of a platform for the sensor component.
-
-### {% linkable_title Installation %}
-
-Copy the code below and create it as a file in `<config_dir>/custom_components/sensor/example.py`.
-
-Add the following to your `configuration.yaml` file:
-
-```yaml
-# Example configuration.yaml entry
-sensor:
-  platform: example
-```
-
-### {% linkable_title Code %}
-
-```python
-from homeassistant.const import TEMP_CELSIUS
-from homeassistant.helpers.entity import Entity
-
-
-def setup_platform(hass, config, add_devices, discovery_info=None):
-    """Setup the sensor platform."""
-    add_devices([ExampleSensor()])
-
-
-class ExampleSensor(Entity):
-    """Representation of a Sensor."""
-
-    def __init__(self):
-        """Initialize the sensor."""
-        self._state = None
-
-    @property
-    def name(self):
-        """Return the name of the sensor."""
-        return 'Example Temperature'
-
-    @property
-    def state(self):
-        """Return the state of the sensor."""
-        return self._state
-
-    @property
-    def unit_of_measurement(self):
-        """Return the unit of measurement."""
-        return TEMP_CELSIUS
-
-    def update(self):
-        """Fetch new state data for the sensor.
-
-        This is the only method that should fetch new data for Home Assistant.
-        """
-        self._state = 23
-```
+<script>
+window.location = 'https://developers.home-assistant.io/docs/en/creating_platform_example_sensor.html';
+</script>