This page of the Resident Evil Village guide provides a list of all the files) to be found near House Beneviento, which is the residence of Donna Beneviento. The following list will help you explore House Beneviento in 100%.
The possibility to find this secret emerges on your way back from House Beneviento to the village, i.e. after dealing with Donna Beneviento.
Ethan has to go through the garden again, but this time you can have a look inside the huts. The diary is inside the hut on the hill, as marked in the picture above. You will find it in a very dark corner near the typewriter.