Add Air Pollutants docs (#7645)

* Add Air Pollutants docs

* Remove some values

source/_components/air_pollutants.markdown

@@ -0,0 +1,27 @@
+---
+layout: page
+title: "Air Pollutants"
+description: "Instructions on how to air pollutants sensors with Home Assistant"
+date: 2018-11-25 08:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+---
+
+The `air_pollutants` gather information about the air quality and pollution details.
+
+The platforms cover the following levels (if they are available):
+
+- The particulate matter 0.1 (<= 0.1 μm) level.
+- The particulate matter 2.5 (<= 2.5 μm) level.
+- The particulate matter 10 (<= 10 μm) level.
+- The Air Quality Index (AQI).
+- The O3 (ozone) level.
+- The CO (carbon monoxide) level.
+- The CO2 (carbon dioxide) level.
+- The SO2 (sulphur dioxide) level.
+- The N2O (nitrogen oxide) level.
+- The NO (nitrogen monoxide) level.
+- The NO2 (nitrogen dioxide) level.
+

source/_components/demo.markdown

@@ -17,6 +17,7 @@ The `demo` platform allows you to use components which are providing a demo of t
 
 Available demo platforms:
 
+- [Air Pollutants]((/components/air_pollutants/) (`air_pollutants`)
 - [Alarm control panel](/components/alarm_control_panel/) (`alarm_control_panel`)
 - [Binary sensor](/components/binary_sensor/) (`binary_sensor`)
 - [Camera](/components/camera/) (`camera`)