[rustdoc] Add option to hide deprecated items

[FeldrinH]

When looking throught the documentation of a library, I generally want to ignore deprecated items. Some libraries (the standard library in particular) never make breaking changes, so they slowly accumulate a layer of deprecated functions that only adds noise to the documentation when working on greenfield projects.

It would be nice if there was an option to hide deprecated items in the docs.rs documentation.

[Saphereye]

@rustbot claim

[FeldrinH]

Similar to the option to hide deprecated items, it might be nice to have an option to hide unstable items, for when you are working with stable Rust. This is std specific since AFAIK other crates can't define unstable items, but still, would be useful.

[Saphereye]

I also wanted to ask whether it would be better for the deprecated item hiding and unstable item hiding options to be available as command line flags, as options in the settings panel, or both.

Edit: The current PR implements this via a command line flag, but I am leaning more toward providing these as just settings panel options, as I feel that should be an easily togglable feature. I would prefer a second opinion though.

[FeldrinH]

I think settings panel options would be better (if it is technically feasible), then you can use the toggles on docs.rs as well.