better 404 page

[roidelapluie]

https://prometheus.io/docs/nothing

We could have a better 404 page, which would keep the header and the navigation, along with link to the latest docs.

[claudiusbr]

Hi.
I had a go at this and created a draft PR for it, only because I'm still not sure about whether what I did with _redirects is the way to go to forward all 404's to the page, and also because I still haven't figured out how to add link to latest docs.

Not sure if Draft PRs are ok, but thought it would be a good way to start. If anyone has the time to have a look at it and give me a few pointers, I'd be grateful.