This is lovely! I'd done something similar in my own setup.

I'm curious what your experience has been with fork/merge – why a monolithic file rather than something more diffuse?

treedocs already has the ability to read multiple treedocs.yaml files. (I should probably document this better.) treedocs.yaml files can be "nested". So you would have one at the top of your repo, and then you could optionally have one or more in subdirectories. Then your top treedocs.yaml is responsible for the repo, but the subdirectory treedocs.yaml files are responsible for their own subdirectory. The idea is that you might have situations where you install a dependency and it clones a subdirectory. Well that subdirectory could just define its own treedocs.yaml.

So it doesn't have to be monolithic.