On this page: https://symfony.com/doc/current/mailer.html#embedding-images

There is a paragraph:

If you want to display images inside your email, you must embed them instead of adding them as attachments. When using Twig to render the email contents, as explained later in this article, the images are embedded automatically. Otherwise, you need to embed them manually.

The "later in this article" is a link, but that link is broken.

Im not sure where the link was intended to go.