Docs: missing working fallback from XDG_DATA_DIRS to XDG_DATA_HOME

The documents only mention the system level (see https://wiki.archlinux.org/title/XDG_Base_Directory 1.2 'system directories') XDG_DATA_DIRS (see https://gitlab.com/search?search=XDG_&nav_source=navbar&project_id=23104371&search_code=true&repository_ref=main), but the code already falls back to XDG_DATA_HOME (see https://gitlab.com/WhyNotHugo/darkman/-/blob/main/scripts.go?ref_type=heads#L24, https://github.com/adrg/xdg?tab=readme-ov-file#xdg-base-directory).

This should likely be added to the documentation, as most users probably run darkman as a systemd user unit rather than a system-wide service, thus darkman needing to look data up from XDG user directories.