Originally pointed out here Docs Preview 404s and answered.
However the same link is also here: https://package.elm-lang.org/help/design-guidelines -> “Write helpful documentation with examples”
Please open an issue at https://github.com/elm/package.elm-lang.org/issues.
Note that there are two online documentation previewers available anyway:
One for Elm 0.19
https://elm-doc-preview.netlify.com/
(or its more powerful local/offline version)
The old official one for Elm 0.18 and older
Issue opened, and thanks for the links, didn’t know about those 
I’ll be trying the offline version later today.