Add a custom 404 page with helpful links

[rok-cesnovar]

Summary:

The docs 404 page could be a bit more helpful. Currently its just the defautl Github Pages 404 error page

Following these steps https://docs.github.com/en/pages/getting-started-with-github-pages/creating-a-custom-404-page-for-your-github-pages-site we could instead create a simple page with links to the user manual, language reference and functions reference?

Current Version:

v2.28.0

[WardBrian]

We currently have 404 pages sitting in each of the folders, but GitHub only looks for a top level one I believe. It would be good to use as a template and add the links you suggest

[mitzimorris]

is there a top-level index.html page?

[mitzimorris]

there's also this: https://github.com/stan-dev/stan-dev.github.io/blob/master/404.md

[rok-cesnovar]

Yeah, that one is triggered at the top level, when calling for example: https://mc-stan.org/foo

https://mc-stan.org/docs/foo leads us to the page I pasted above, because /docs is routed to this repo I guess.

[WardBrian]

You can view the page bookdown generates for this by manually navigating to https://mc-stan.org/docs/2_28/stan-users-guide/404.html for example. It would be great if github was smart enough to use these