A document that links to another, for rendering "referenced by" panels.
Backlinks are the reverse of the links inside a document: given a page, which other pages point at it. They are what lets a reader landing on a low-level module page find the guide that explains when to use it.
DocShell does not compute backlinks. Resolving a link in a document to the entry it refers to needs a routing scheme and a naming convention, and both belong to the host. What DocShell provides is the shape those relationships take on the wire, so that a graph-backed producer and a renderer agree without a private contract between them.
The backlinks key is optional in DocShell.Presentation.Source.presentation/0
precisely because most producers have nothing to put there. When present it
maps an entry id to the entries referencing it, and
DocShell.Presentation.GraphProjector.validate/1 checks the shape.
Only enough is carried to render a link — id, title, path. A renderer wanting more looks the referencing entry up in the navigation index.