Exclude the contents of a directory *except* for the index file

[zackbatist]

Description

I want to be able to ignore rendering all content situated within a directory's subdirectories, but I do not want to specify the names of those subdirectories in the _quarto.yml (the names of the directories imply their identity, which I'm trying to shield, so having them named in the main config is a potential leak).

Is there a way to state a preference to ignore the contents of a directory except certain files or directories? Since you are already treating certain files as exceptional (readme files are excluded by default), maybe this could be extended to index files too. Alternatively, it could be nice to have the flexibility to flip the paradigm from a blacklist to a whitelist, on a directory-by-directory basis.

I realize that this is presently not supported (implied in the Project Basics documentation) but this flexibility would be really helpful!