Add flag to exclude deprecated items in rustdoc

[Saphereye]

Fixes: #149483

This PR introduces a flag that hides all deprecated items when generating docs.

For example, given the following code:

#![allow(unused)]
pub struct Visible;

#[deprecated(since = "1.2.3", note = "use Visible instead")]
pub struct OldType;

pub mod m {
    #[deprecated]
    pub fn old_fn() {}
    pub fn new_fn() {}
}

#[deprecated]
pub fn legacy_api() {}

pub fn modern_api() {}

And on generating the docs:

Original	After Edits
	but the symbols still exists, so its still searchable

Edits:

• changed behavior from removing symbols to just hiding them

[rustbot]

r? @GuillaumeGomez

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[GuillaumeGomez]

Instead of not generating items, I think adding a setting to hide them would be better.