Auto merge of #119066 - notriddle:notriddle/sidebar-source-redesign, r=GuillaumeGomez

rustdoc: clean up source sidebar hide button

This is a redesign of the feature, with parts pulled from rust-lang/rust#119049 but with a button that looks more like a button and matches the one used on other sidebar pages.

Preview:

* http://notriddle.com/rustdoc-html-demo-8/source-sidebar-resize/src/std/lib.rs.html
* http://notriddle.com/rustdoc-html-demo-8/source-sidebar-resize/std/index.html

| | Before | After |
|--|--|--|
| Closed | ![image](https://github.com/rust-lang/rust/assets/1593513/092bed75-79c3-412f-8e7b-557f30dfb1e3) | ![image](https://github.com/rust-lang/rust/assets/1593513/b68e1ee9-9aef-484d-a5b1-2fd29c9d72ea)
| Open | ![image](https://github.com/rust-lang/rust/assets/1593513/95cf9545-25b1-48ec-820b-02e1aec99839) | ![image](https://github.com/rust-lang/rust/assets/1593513/923532f6-59e0-4d7c-9976-21699c30d42e)
| Mobile Closed | ![image](https://github.com/rust-lang/rust/assets/1593513/9bc00cc5-937c-4120-94be-94c7cb6d5297) | ![image](https://github.com/rust-lang/rust/assets/1593513/76a744d8-aac2-46fe-abb9-3b34e2d3ccaa)
| Mobile Open | ![image](https://github.com/rust-lang/rust/assets/1593513/d19a94fe-47b1-462d-a280-44fc215b9b72) | ![image](https://github.com/rust-lang/rust/assets/1593513/2b2e3dec-b610-4b12-8a72-35b86359ba45)

Author: bors