Function preprocess_link 

fn preprocess_link(
    ori_link: &MarkdownLink,
    dox: &str,
) -> Option<Result<PreprocessingInfo, PreprocessingError>>

Returns:

• None if the link should be ignored.
• Some(Err) if the link should emit an error
• Some(Ok) if the link is valid

link_buffer is needed for lifetime reasons; it will always be overwritten and the contents ignored.