"Hide files" toggle should be fixed so it isn't hidden after scrolling

[jyn514]

The "Hide files" button doesn't scroll with the content, so you can end up having to scroll back up to the top to collapse the sidebar:

Rustdoc does this by making the sidebar scrolling independent from the content scrolling, maybe we could copy that? Or would you prefer to leave that for a follow-up?

cc @GuillaumeGomez
Originally posted by @jyn514 in #1464 (comment)

[GuillaumeGomez]

I also think the scrolling should be independant (in case we count votes haha).