Previously we only logged a warning if we attempted to auto-link
to a doc that had no `path` property, which implies that it is private
and is not rendered.
Now we fail hard during full doc-gen, although when running the
`yarn serv-and-sync` command it should not fail if changes are
only made to guides, which is what authors who use this tool
are most likely to do.
PR Close#40404
a7a2123ade