Also check latest release for Custom 404 page

[magenta404]

The current 404 resolution strategy is:

Firstly, attempt to serve the 404 of the current version (version_slug)
Secondly, try to serve the 404 page for the default version (project.get_default_version())

Would it be possible to fallback to the latest version if the default version doesn't have a 404 page? Our default version is stable, but the 404 is on latest which made me wonder why it wasn't working in all cases.

It would also help if the resolution strategy was documented.

[humitos]

Would it be possible to fallback to the latest version if the default version doesn't have a 404 page?

This makes sense to me. We would need to think a little more about this since our 404 checks are already super slow (we hit storage backend multiple times) and this sometimes causes us some problems. Adding an extra check here will definitely increase the times we will hit it.

I already attempted to fix this problem in #6797 but we finally took another direction. The issue is not fixed and that PR would definitely help, but it also increases complexity 🤷

It would also help if the resolution strategy was documented.

This is something we definitely should do in https://docs.readthedocs.io/en/stable/hosting.html#custom-not-found-404-pages -- cc @astrojuanlu

[magenta404]

Thanks and understandable. Another additional (or separate) improvement for consideration is to allow admins to choose which fallback should take priority between stable and latest. Since the fallback is global, that choice could be useful (I would find it useful as I would prefer to use latest for the fallback 404 page after implementing what I needed to, I don't think it will be that important for myself. Might still be a good option).

[astrojuanlu]

We clarified the documentation about the current strategy. I propose to leave the issue open to discuss any changes.