Rollup merge of #93183 - jsha:mobile-nav-fixes, r=GuillaumeGomez

Rollup merge of #93183 - jsha:mobile-nav-fixes, r=GuillaumeGomez

rustdoc: mobile nav fixes

- Make sure the mobile-topbar doesn't overflow its height if the user sets a bigger font.

- Make sure the sidebar can be scrolled all the way to the bottom by shortening it to accommodate the mobile-topbar.

- Make the item name in the mobile-topbar clickable to go to the top of the page.

- Remove excess padding sidebar in mobile mode.

Demo https://rustdoc.crud.net/jsha/mobile-nav-fixes/std/string/struct.String.html

r? `@GuillaumeGomez`