Rustdoc: Sidebar toggle is not accessible

[tazjin]

The sidebar toggle on Rustdoc pages is not recognised as a clickable item by accessibility tooling, making it difficult to toggle in circumstances where mouse usage is difficult.

This is an example of what it currently looks like in Vimium, a common extension for making pages keyboard-accessible:

Other than that, most Rustdoc features seem to be navigable just fine - thanks for that :)