Rollup merge of #139562 - notriddle:notriddle/ew-resize, r=GuillaumeGomez

rustdoc: add a handle that makes sidebar resizing more obvious

This aims to make the resizable sidebars more obvious

Preview: <https://notriddle.com/rustdoc-html-demo-12/sidebar-resize-handle/std/index.html>

![image](https://github.com/user-attachments/assets/d4d70982-8045-4fed-818a-982108b0d3b3)

![image](https://github.com/user-attachments/assets/4aaa3663-19f3-4e04-89c6-53db0ddb72ed)

![image](https://github.com/user-attachments/assets/3f612c5b-6be1-4383-801a-067c87425eb9)

This change is based on some discussion on [lolbinarycat's idea], but with a more "traditional" design. Specifically, while very few systems use exactly this design, most of them use [a skeumorph](https://ux.stackexchange.com/questions/80463/what-do-the-3-close-horizontal-bars-not-hamburger-menu-represent-and-what-is-t/80591#80591) of a grip texture:

- This design is similar to the one used in the Rust Playground, and almost identical to UX StackExchange:

  <details><img src="https://github.com/user-attachments/assets/39a6bb69-4895-4fd0-87da-b87913bc7309"></details>

  <details><img src="https://github.com/user-attachments/assets/a41942e1-651b-410b-b855-2aafe8fe54f4"></details>

- In Jira, resizable sidebars have a stack of four dots, but only in one row.

  <details><img src="https://github.com/user-attachments/assets/13047998-02bf-47e6-b796-16f393f870b0"></details>

- In The GIMP, resizable sidebars have a stack of three dots.

  <details><img src="https://github.com/user-attachments/assets/138f5c21-3069-4bbe-b306-0bb9a4bf0318"></details>

- In [old Windows], "panes" are defined to have the same border style as a window, which has a raised appearance. To evoke this, the PR adds a lightweight "shadow" border, darker than the sidebar itself

  <details><img src="https://github.com/user-attachments/assets/301da4b8-6c48-4131-b741-1689af84670a"></details>

- In [NeXT], a drag point usually had an innie, whether the line in a slider or the circle in a scroller; I can also hide and show the favorites bar in Workspace by dragging on a circular "grip spot"

  <details><img src="https://github.com/user-attachments/assets/b13c2d30-a3a8-4672-90fa-58c1fdf19f42"></details>

- In [old Mac], drag handles for things usually had a "grip track" of parallel lines.

  <details><img src="https://github.com/user-attachments/assets/1fbecc67-ffbc-4ed6-a8c5-a9ff085638db"></details>

  *This design is far closer to old Mac than anything else*, though they've put it in the bottom corner instead of the middle.

- [OSX] kept that, but the "Source List" part of the Finder still had the circle grip for a time the same way Workspace did (resulting in an odd mishmash, if you compare the source list sidebar with the other grip tracks embedded in the scrollbars).

  <details><img src="https://github.com/user-attachments/assets/551b8f9d-2dd8-4291-917f-dc88741a2b97"></details>

[lolbinarycat's idea]: https://github.com/rust-lang/rust/pull/139420
[old Windows]: https://archive.org/details/windowsinterface00micr/page/n9/mode/2up
[old Mac]: https://archive.org/details/apple-hig/1996_Human_Interface_Guidelines_for_Mac_OS_8_%28WWDC_Release%29/page/16/mode/2up
[NeXT]: https://archive.org/details/apple-hig/1993%20NeXTSTEP%20User%20Interface%20Guidelines%20-%20Release%203/page/145/mode/2up
[OSX]: https://dn721903.ca.archive.org/0/items/apple-hig/MacOSX_HIG_2005_09_08.pdf#page=267

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