iMac as external display for new MacBook Pro?
Target display mode is supported on your iMac and how to use it is described in this support document on the Apple site.

It states that for the Mid 2011 iMac, which you linked to, a Thunderbolt cable will work (Thunderbolt wraps the DisplayPort data stream)


  iMacs you can use | Cable       | Port you need
  as a display      | you need    | on your other Mac
  ----------------------------------------------------
  iMac (Mid 2011)   | Thunderbolt | Thunderbolt
  
  Use Target Display Mode
  
  Follow these steps to start using your iMac as a display:
   1. Make sure both computers are turned on.
   2. Connect the Mini DisplayPort or
  Thunderbolt cable between the two computers.
   3. Press Command-F2 on the keyboard of the iMac that you want to use as a display.  
  
  After you press Command-F2, your iMac displays the desktop from
  the connected Mac.


So, you should be able to connect a new MacBook Pro via a Thunderbolt 3 -> Thunderbolt 2 adapter and a standard Thunderbolt 2 cable.