add ups docs.

docs/extras/ups.md

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+# Clean shutdown using a UPS
+
+Usually the operating system is required to shutdown in order to avoid data loss and a corrupted OS. Often this is neglected and the power is just cut by removing the power cord from the photobooth.
+
+Following is a recommendation how to implement a UPS. The features of the reference setup are:
+
+- automatic power up when the power cord is connected to the photobooth (auto power on)
+- automatic shutdown on unplugging the power cord by power loss detection
+- cut power when shutdown finished to turn off the UPS protecting the batteries and save power
+
+## Reference setup
+
+| UPS | Pi supported | Auto power on | Auto shutdown | Cut power | Notes |
+| --- | - | ---- | ---- | ---- | -- |
+| Geekworm X728 | Pi 3,4,5 | ✅ | ✅ | ✅ | - |
+
+You tested another setup? [Please tell us all the details so we can add the solution here!](https://github.com/photobooth-app/photobooth-app/discussions)
+
+## Software setup
+
+### Geekworm X728
+
+There is no need to use the custom Geekworm software at all. Just set the following parameters in the OS config.txt.
+
+```ini title="/boot/firmware/config.txt"
+[all]
+# Settings for Geekworm x728 v2.5
+# hardware clock (remove fake-hwclock package when enable following line)
+dtoverlay=i2c-rtc,ds1307
+# cut power after shutdown
+dtoverlay=gpio-poweroff,gpiopin=26,timeout_ms=6000 # early x728 versions is GPIO 13
+# shutdown automatically on power loss detected, wait 3s before shutdown.
+dtoverlay=gpio-shutdown,gpio_pin=6,active_low=0,gpio_pull=down,debounce=3000
+# report battery status from x728 to the OS
+dtoverlay=i2c-sensor,max17040
+```

mkdocs.yml

@@ -121,6 +121,7 @@ nav:
       - extras/sync.md
       - extras/wifi.md
       - extras/wled.md
+      - extras/ups.md
 
 not_in_nav: |
   /extras/shareservice-landing.md