Add screenshot to locate the Disks menu in the top navigation bar (#15669)

During the user testing, everybody who looked for it had a hard time
finding it or even gave up.

wiki/src/doc/encryption_and_privacy/veracrypt.mdwn

@@ -139,6 +139,8 @@ Using <span class="application">Disks</span>
      <span class="guimenuitem">Attach Disk Image&hellip;</span></span> from the
      top navigation bar.
 
+     [[!img disks-menu.png link="no" alt=""]]
+
 1. In the <span class="button">Select Disk Image to Attach</span> dialog:
 
    - Unselect the <span class="guilabel">Set up read-only loop device</span>