rustdoc: Improve the navigation experience

[ishitatsuyuki]

In the current situation, users navigates either with links from the overview, or by collapsing all descriptions and searching through what they want.

They both have their own problems:

And I often resort to "Find in page" which is bad on accuracy.

We need a better navigation system, and in either way we should ditch the collapse feature. I have two ways in mind:

• Use a right sidebar. Sidebar allows you to collapse things by group not per item. It also has a smaller font size, allowing more items to fit in the screen.
• Make the brief link to everything available in the page. Rust pseudocode is easier to read for some people (including me).

To compare these two approaches, sidebar is more suitable for continuously navigating items without caring the backstack, while the brief is more like "navigate, read, back, navigate, read, back".

Related: #39952

[GuillaumeGomez]

We don't have enough space available for a second sidebar. Also, I'm not sure what you proposed is better than the current version. To be clear: I agree on the fact that some UI refactoring would be welcome, but those ones aren't convincing me...