Not possible to link internally to a Details box

[ptgott]

It is possible to navigate directly to a Details box, and open the box, by entering the title of the details box as a URL fragment, e.g., https://goteleport.com/docs/management/join-services-to-your-cluster/join-token/#connecting-directly-to-the-auth-service.

However, if you add a relative link to the same Details box fragment, e.g., ../join-services-to-your-cluster/join-token/#connecting-directly-to-the-auth-service, the docs linter fails, indicating that the target heading does not exist.

We should make it possible to link to Details boxes within the docs by adjusting our linter logic.