Function generate_item_pages 

pub fn generate_item_pages(
    package: &DocPackage,
    output_dir: &Path,
) -> Result<()>

Generate individual RST pages for each item (class, function, type alias, variable)

Item pages are placed in _items/ subdirectory to avoid filename collisions with module pages (e.g., a class pkg.Foo vs submodule pkg.Foo).

The _items/ directory is cleared before generation to remove stale pages from renamed or deleted items.