I just booked one today for week after next. On the discoverqatar.qa website I clicked on the stopover tab (not the hotel offers as said no availability) then put in the date and gives you selection 4/5*. Not the best choice but got the grand hyatt (which looks good although 25 mins away) for circa £20 admin fee. You just enter your ticket number in the booking page.
Note I hadn't booked as a multi-city just had an o/night as part of an R class (discounted bus) ticket. And I had booked the flight about a month ago. I guess I will get free-visa on arrival and grab an uber.
I was going to book the Sharq village as so close and have stayed before - but was £180 on hotels.com so this is a great deal.