rustdoc: improve source navigation display on mobile

[jsha]

Following up on #102815 (which I think was correct since we're not currently showing the topbar).

Here's what a source page looks like on my Pixel 6a:

The Rust logo really shouldn't wrap to the previous line, and the expander for the sidebar overlaps awkwardly.

One possible fix: we should actually use the mobile topbar on source pages, like so:

This provides three things:

• the hamburger icon could open the source sidebar, and follows the scroll just like the current > icon
• the Rust logo provides navigation back to the doc root
• the "lib.rs" text provides context for where the reader is currently, and also provides navigation back to the top of the page (where the search bar is)

/cc @GuillaumeGomez @notriddle

[notriddle]

The downside of the top bar is that it makes the already tiny viewport even smaller. When reading source code on mobile, you need all the screen real estate you can get.

[GuillaumeGomez]

We could do like on the other pages: the top navbar remains at the very top of the page (not of the screen) and when we press the button (hamburger one?), we show the menu. That would make things more coherent.