rustdoc: indicate unstable items in search results (and sidebar?)

[RalfJung]

Inspired by discussion on IRLO: Consider these search results.

It is impossible to tell that half of these are stable and can be used by everyone, while the other half is unstable. I think it'd be good to have some sort of indication for that.

The same goes for the sidebar.

[GuillaumeGomez]

Yeah that makes sense. Funnily enough, we're discussing a feature to hide unstable items from docs 9with a JS setting) which would require to have this information in sidebar/search results.

[lolbinarycat]

@rustbot claim

[lolbinarycat]

Once #149460 is merged, search js will have enough info to do this easily.