That only works for directly building a Zig file (e.g. zig build-exe bla.zig -femit-docs), but not for projects with a build.zig (e.g. try zig build-exe -h | less and search for ‘docs’ you’ll find the -femit-docs option, but not for zig build -h in a Zig project).
You’ll basically need to add a build step to build.zig to generate docs.
PS: e.g. see: How to properly make a static library and docs