docs: document on-created-empty workspace rule (#375)

the new workspace rule is being introduced in hyprwm/Hyprland#3559

pages/Configuring/Workspace-Rules.md

@@ -33,11 +33,14 @@ workspace=DP-1,3,rounding:false,bordersize:0
 | rounding:[b] | Whether to draw rounded windows or not | bool |
 | decorate:[b] | Whether to draw window decorations or not | bool |
 | persistent:[b] | Keep this workspace alive even if empty and inactive | bool |
+| on-created-empty:[c] | A command to be executed once a workspace is created empty (i.e. not created by moving a window to it). See the [command syntax](../Dispatchers#executing-with-rules) | string
 
 ### Example Rules
 ```ini
 workspace = 3, rounding:false, decorate:false
 workspace = name:coding, rounding:false, decorate:false, gapsin:0, gapsout:0, border:false, decorate:false, monitor:DP-1
 workspace = 8,bordersize:8
 workspace = name:Hello, monitor:DP-1, default:true
+workspace = 5, on-created-empty:[float] firefox
+workspace = special:scratchpad, on-created-empty:foot
 ```