Migrate "Create a Custom-Search Extension" wiki page
Created by: seancolsen
We didn't catch this wiki page during our migration project because it's not in the CRMDOC
space: https://wiki.civicrm.org/confluence/display/CRMDOC46/Create+a+Custom-Search+Extension
But it's referenced here: https://docs.civicrm.org/dev/en/latest/extensions/civix/#building-your-custom-search
I think we should migrate it to the Dev Guide, at least in-part.