Function add_keyed 

pub fn add_keyed(
    root: &mut Value,
    prefix: &str,
    slug: &str,
    keyed_lists: &[KeyedList<'_>],
    seed: Option<&Value>,
    fields: &[(String, Value)],
) -> DocumentResult<()>

Add a new element to a keyed list.

The new element is built in three layers:

1. seed fields (if provided) — default template values
2. { slug_field: slug } — always set, overrides any slug value in seed
3. explicit fields — override both seed and slug (except the slug field)