Support '/' shortcut to search

[aDotInTheVoid]

Problem

In rust-lang/rust#123355, we added / as an additional shortcut to open the searchbar. It's was already used by github, zulip, less, and google drive to focus/open the searchbar. Given that it's so common, I now find myself typing / by mussle memory, and expecting it to work in mdbook, before remembering that I need to use s here.

Proposed Solution

/ should behave like s, and open the searchbar.

Notes

I'd be happy to contribute an implementation of this, assuming you're willing to accept the feature.