* Add index for new Xiaomi ClearGrass E-Ink Sensor
* Add Xiaomi Cleargrass Sensor Documentation
Add all the needed documentation, such as index,
references and main page of this sensor
* Fix images width
Adjust images width to a better fit in the page
* Fix apiref of Xiaomi Sensors
The apiref of Cleargrass and LYWSD02 models point
to the mijia's api, so I fix this
* Fix Xiaomi Cleargrass documentation mistake
Line 43 of xiaomi_cleargrass.rst references to
Xiaomi Mijia's sensor. It should reference to
Xiaomi Cleargrass.
Also I change titles of this sensor in order to
avoid the title uses 2 lines
