Add Dexcom integration documentation (#12777)

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

source/_integrations/dexcom.markdown

@@ -0,0 +1,58 @@
+---
+title: Dexcom
+description: Instructions on how to integrate your Dexcom CGM data into Home Assistant.
+ha_category:
+  - Sensor
+ha_release: 0.113
+ha_iot_class: Cloud Polling
+ha_config_flow: true
+ha_codeowners:
+  - '@gagebenne'
+ha_domain: dexcom
+---
+
+The Dexcom integration allows you to view your CGM data from [Dexcom](https://www.dexcom.com/) in Home Assistant.
+
+## Configuration
+
+<div class='note warning'>
+Some people have had problems with connecting when their Dexcom passwords are entirely numeric. If you have connection issues in that case, try changing your password to something with a mix of numbers and letters.
+</div>
+
+You will need to set up the [Dexcom Share](https://provider.dexcom.com/education-research/cgm-education-use/videos/setting-dexcom-share-and-follow) feature in your Dexcom G6 App to use this integration. Once you have done that, perform the following steps.
+
+To add `Dexcom` to your installation, go to **Configuration** >> **Integrations** in the UI, click the button with `+` sign and from the list of integrations select **Dexcom**.
+
+#### Server
+
+There are two Dexcom Share servers, `US` for United States customers, and `OUS` for all customers outside of the United States.
+
+#### Unit of measurement
+
+The integrations allows both `md/dL` and `mmol/l` units of measurement for blood glucose values. To change your preferred unit of measurement, got to **Configuration** >> **Integrations** in the UI, and click `OPTIONS`.
+
+### Sensor
+
+If you have a sensor session running, and once you have enabled the Dexcom integration, you should see the following sensors:
+
+- Blood glucose value sensor
+- Blood glucose trend sensor
+
+### 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.dexcom_YOUR_USERNAME_glucose_value
+    platform: numeric_state
+  condition: time
+    after: '22:00:00'
+    before: '06:00:00'
+  action:
+  - service: light.turn_on
+      data:
+        entity_id: light.kitchen
+```