docs: document download_perc and install_perc MQTT topics

Author: NirKli

website/docs/integrations/mqtt.md

@@ -18,6 +18,8 @@ Vehicle data will be published to the following topics:
 | `teslamate/cars/$car_id/version`                       | 2019.32.12.2                                                                                | Software Version                                                                      |
 | `teslamate/cars/$car_id/update_available`              | false                                                                                       | Indicates if a software update is available                                           |
 | `teslamate/cars/$car_id/update_version`                | 2019.32.12.3                                                                                | Software version of the available update                                              |
+| `teslamate/cars/$car_id/download_perc`                 | 100                                                                                         | Download progress of the available update in percent                                  |
+| `teslamate/cars/$car_id/install_perc`                  | 42                                                                                          | Installation progress of the update in percent                                        |
 |                                                        |                                                                                             |                                                                                       |
 | `teslamate/cars/$car_id/model`                         | 3                                                                                           | Either "S", "3", "X" or "Y"                                                           |
 | `teslamate/cars/$car_id/trim_badging`                  | P100D                                                                                       | Trim badging                                                                          |