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

rustbot has assigned @GuillaumeGomez.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[GuillaumeGomez]

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

[Saphereye]

Added a flag to hide unstable items (based on feedback in the parent issue). I also tried adding a corresponding option in the settings panel, but ran into two issues:

1. Not all elements can be reliably identified as unstable or deprecated via existing tags, so they still appear in places like the sidebar. I experimented with injecting extra metadata into the generated nodes, but it felt hacky and would require adding the metadata to many places. In the future if we were to add more hiding flags this would bloat the docs(?)

2. I couldn’t make the settings option persist and apply responsively across pages. Enabling it on one page wouldn’t carry over to the next, and elements only refreshed to the hidden state after reopening the settings panel.

I will check this out more but will keep the flags for now.