Add doc for Nightscout integration (#14185)

Co-authored-by: Franck Nijhof <git@frenck.dev>

source/_integrations/nightscout.markdown

@@ -0,0 +1,49 @@
+---
+title: Nightscout
+description: Instructions on how to integrate your Nightscout CGM data into Home Assistant.
+ha_category:
+  - Sensor
+ha_release: 0.115
+ha_iot_class: Cloud Polling
+ha_config_flow: true
+ha_codeowners:
+  - '@marciogranzotto'
+ha_domain: nightscout
+---
+
+The Nightscout integration allows you to view your CGM data from [Nightscout](http://www.nightscout.info/) in Home Assistant.
+
+## Configuration
+
+To add `Nightscout` to your installation, go to **Configuration** >> **Integrations** in the UI, click the button with `+` sign and from the list of integrations select **Nightscout**.
+
+Then just input your server URL in the `Host` field. For example:
+
+- `https://localhost:5423`
+- `https://your-app-name.herokuapp.com`
+
+### Sensor
+
+If you have a sensor session running, and once you have enabled the Nightscout integration, you should see a `sensor.blood_glucose` entity.
+
+It's icon changes to indicate the direction or trend, of the glucose readings.
+The state is the last reading from Nightscout, and you can see other information about the reading in the sensor's attributes.
+
+### Example Automation
+
+```yaml
+- id: '1234567890123'
+  alias: overnight_low_kitchen_lights
+  description: Turn on the lights in the kitchen if my blood sugar drops low overnight
+  trigger:
+  - below: '65'
+    entity_id: sensor.blood_glucose
+    platform: numeric_state
+  condition: time
+    after: '22:00:00'
+    before: '06:00:00'
+  action:
+  - service: light.turn_on
+      data:
+        entity_id: light.kitchen
+```