Add an example template to extract air quality (#18869)

* Add an example template to extract air quality 

Thought it would be helpful to add a example template showing how to extract air quality and capitalise the results. 

This is my first PR for this project so I apologise if I've made any mistakes

* Use new template format & changed to state_attr()

* Tweaks

* Tweak

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

source/_integrations/vesync.markdown

@@ -126,5 +126,17 @@ sensor:
         value_template: '{{ states.switch.vesync_switch.attributes["voltage"] | float }}'
         unit_of_measurement: "V"
 ```
+{% endraw %}
+
+Extracting air quality from a VeSync LV-PUR131S air purifier. Change the `vesync_air_purifier` to match your device's entity ID.
+
+{% raw %}
+
+```yaml
+template:
+  - sensor:
+      - name: "VeSync Air Quality"
+        state: "{{ state_attr('fan.vesync_air_purifier', 'air_quality') | title }}"
+```
 
 {% endraw %}