Added an example in the user guide on using operations on 2d elements. #2316
This is in response to issue #1108. Even though the documentation has improved massively since I first started using Holoviews, I was having some trouble applying operations to Images recently, so I thought it might be nice to have an example of this in the documentation. I also think it's neat to show the ability to apply an operation to a container in this section, which is an amazing and convenient feature, imo. Just let me know what you think.