rustdoc: toggle to show private items in rustdoc

[lolbinarycat]

requested by rust4linux, will be an opt-in nightly only feature at first.

the core bit of code would be something like document.querySelectorAll("dt:has([title='Restricted Visibility'])").forEach(x => x.hidden = true) on a button/toggle. we probably want to add a css class for hidden items.

how it interacts with the search index is complicated, so the first MVP will likely not affect rustdoc search at all

[lolbinarycat]

perhaps this should be a settings checkbox?

[mejrs]

I'd recommend that they host their own documentation but with everything shown. See https://pyo3.rs/internal/doc/pyo3/index.html for an example.

[lolbinarycat]

@mejrs I believe that's close to what they're already doing.