Stray's vision of the future is populated by a number of robots, many of which help the cat at the center of its story along its adventure. Four robots in particular carry special importance: a little group known as the Outsiders. They're as committed to escaping the city as the cat protagonist, and their notes on the subject are a valuable resource on that quest.
All four notebooks are located in the Slums. Here's where to find Doc's.
How to Find Doc's Notebook
You'll want to start at Clementine's apartment for this excursion. Head outside and take a left, clambering over to the couch sitting on the roof. Facing the sofa, you should see the entrance to Doc's apartment. (There's also a remote you can play with here to futz with whatever's on the TV.)
Head into Doc's apartment and into the small room at the back of the place. The entrance is blocked by books; get through them as you wish. On Doc's bed, you should see a note and a set of keys; grab both. The note will tell you there's a hidden safe behind a particular stack of books. Knock aside the books in front of the bottom row of the shelves labeled L to M. You'll find the safe behind it, and you can use the key you picked up on Doc's bed to open it and grab his notebook.