docs: add Windows PowerShell hook examples (#567)

Author: homandr

docs/content/3.cookbooks/1.command-hook-examples.md

@@ -145,3 +145,67 @@ fi
 ```
 
 **Error Behavior:** `ON_ERROR_CANCEL`
+
+### Windows PowerShell examples
+#### GUI message box on error condition
+The message box stays on screen until acknowledged by the user. Use the same script for both backup plan and repo settings to catch errors.
+
+**Condition** `CONDITION_ANY_ERROR`
+
+**Script**
+```powershell
+Add-Type -AssemblyName System.Windows.Forms
+# This option puts message box on top of all windows.
+$options = [System.Windows.Forms.MessageBoxOptions]::ServiceNotification
+$defbutton = [System.Windows.Forms.MessageBoxDefaultButton]::Button1
+$buttons = [System.Windows.Forms.MessageBoxButtons]::OK
+$icon = [System.Windows.Forms.MessageBoxIcon]::Error
+$title = "Backrest"
+$message = '{{ .Summary }}'
+[System.Windows.Forms.MessageBox]::Show($message, $title, $buttons, $icon, $defbutton, $options)
+
+# Include this comment and the line above it. See https://github.com/garethgeorge/backrest/issues/400
+```
+#### GUI toast warning notification
+Toast or app notifications appear for a short time before disappearing without user acknowledgement.
+
+**Condition** `CONDITION_SNAPSHOT_WARNING`
+
+**Script**
+```powershell
+Add-Type -AssemblyName System.Windows.Forms
+$balloon = New-Object System.Windows.Forms.NotifyIcon
+$balloon.Icon = [System.Drawing.SystemIcons]::Warning
+$balloon.BalloonTipIcon = [System.Windows.Forms.ToolTipIcon]::Warning
+$balloon.BalloonTipText = '{{ .Summary }}'
+$balloon.BalloonTipTitle = "Backrest"
+$balloon.Visible = $true
+$balloon.ShowBalloonTip(5000)
+Start-Sleep -Seconds(5)
+$balloon.Visible = $false
+$balloon.Icon.Dispose()
+$balloon.Dispose()
+
+# Include this comment and the line above it. See https://github.com/garethgeorge/backrest/issues/400
+```
+#### GUI toast information notification
+
+**Condition** `CONDITION_SNAPSHOT_SUCCESS`
+
+**Script**
+```powershell
+Add-Type -AssemblyName System.Windows.Forms
+$balloon = New-Object System.Windows.Forms.NotifyIcon
+$balloon.Icon = [System.Drawing.SystemIcons]::Information
+$balloon.BalloonTipIcon = [System.Windows.Forms.ToolTipIcon]::Information
+$balloon.BalloonTipText = '{{ .Summary }}'
+$balloon.BalloonTipTitle = "Backrest"
+$balloon.Visible = $true
+$balloon.ShowBalloonTip(5000)
+Start-Sleep -Seconds(5)
+$balloon.Visible = $false
+$balloon.Icon.Dispose()
+$balloon.Dispose()
+
+# Include this comment and the line above it. See https://github.com/garethgeorge/backrest/issues/400
+```