rustdoc: add a handle that makes sidebar resizing more obvious

This change is based on some discussion on [lolbinarycat's idea],
but with a more "traditional" design. Specifically, this is the
closest thing I could find to a consensus across many systems I
looked at for inspiration:

- In Jira, resizable sidebars have a stack of four dots.
- In The GIMP, resizable sidebars have a stack of three dots.
- In [old Windows], "panes" are defined to have the same border
  style as a window, which has a raised appearance.
- 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"
- In [old Mac], drag handles for things usually had a "grip track"
  of parallel lines.
- [OSX] kept that, but the "Source List" part of the Finder still had
  the circle grip for a time the same way Workspace did

[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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