Documenting current state of isolating icecat

[Christopher Lemmer Webber]

If you want to have the hairs on the back of your neck stand up and not
lie down for a while, why not read this:

  
https://robertheaton.com/2019/06/24/i-was-7-words-away-from-being-spear-phished/

Previously there were some threads about isolating icecat and other
graphical applications:

  https://lists.gnu.org/archive/html/help-guix/2018-01/msg00056.html

However, it's not obvious to me if we have an answer yet on what to do.
Whatever it is, I'd like to do it.

Could someone point me in the right direction?  Much appreciated.
Maybe we should even include it as an example in the documentation?
That could be a big win, and make it clearer to users how Guix comes
with tools that can help empower them.

 - Chris