Function write_qlink_json 

pub fn write_qlink_json(
    storage_root: &Path,
    url: &str,
    name: &str,
    description: &str,
    ingested_to_library: bool,
    context_assertions: Option<Vec<Value>>,
) -> Result<(String, PathBuf), String>

Persist a qlink JSON document (always succeeds if disk allows).