Ensure 404 page is visible for all wrong URLs

[huonw]

From #4:

A "404" shows the menu structure but no actual content: https://docs.pantsbuild.org/xyz/abc. It'd be good to have some sort of real content describing the problem.

[huonw]

Ah, broken URLs under a version do show a useful 404: https://docs.pantsbuild.org/2.19.x/abc ... so this is more about making sure that's always shown.

[thejcannon]

Also, for reference here's the existing 404 page: https://www.pantsbuild.org/docs/foo

[thejcannon]

OK so spinning up a new docs site, it does 404 correctly. So far I haven't been able to deduce what's different about our little slice of heaven and a fresh docs site that'd cause this.

[thejcannon]

OK I've narrowed it down to what appears to be "disableVersioning: true" making the top-level 404 page be blank...

which is weird because that should be false on prod...

[thejcannon]

Alright, so poking at it more, this just looks like a Docusuarus bug: facebook/docusaurus#9688