docs: remove link to demo firmware

Author: sayanee

index.md

@@ -14,10 +14,6 @@ downloads:
   bom: true
   freecad: false
   case: false
-software:
-  - name: Demo firmware
-    code: _code/demo
-
 features:
   - detail: Read UV Index with the on-board sensor <code>VEML6075</code> and the battery level
   - detail: Display the UV Index color code with the on-board RGB LED