Support slash `/` keyboard shortcut to focus search

[SUPERCILEX]

/ is ubiquitous everywhere as the shortcut to focus search (every google product, github, etc).

[syphar]

Thank you for the idea!

This is a bigger question since S is the shortcut that is used inside rustdoc, and so also on the docs.rs homepage to focus the crate search.

IMO docs.rs would follow rustdoc here, so this is a question that should be raised there.

I found a partially related issue in rust-lang/rust#44964, though you could also create a new issue for it.

[SUPERCILEX]

Oh, I thought this was the rustdoc repo too. Anyway, yeah the / shortcut should be everywhere rust presents a search bar.

[syphar]

It seems like rustdoc will add slash : rust-lang/rust#123355

Which means we could too, when the pr above is merged.