[Documentation Request] Please expand qmk userspace documentation to more thoroughly describe what can be overlaid and/or added to individual keyboard firmware, and also describe limitations.

[Steve973]

Description

I am trying to understand exactly what can be overlaid in terms of keyboard files in the qmk userspace repo. Since the qmk.json file specifies build targets by the specific keymap level, it seems like the overlay would consist only of files that would exist in a specific keymap directory. Since the repo documentation in README.md is not specific about this, that is only an assumption. Some explanation about the scope of a build target overlay would be very helpful for people using this userspace repo. Though I am doubtful about the ability to do this with the userspace overlay, is there any way to overlay any files outside of a keymap directory? For example, if a config.h file exists at the keyboard level, it will affect the keymap, so if this is either possible, or impossible, it would be helpful to understand this, as well.