"I didn't know that the sidebar could be resized"

[notriddle]

Problem

rust-lang/rust#115660 (comment)

Based on usability discussions about adding this same feature to rustdoc, at least two expert users didn't know you could resize the navigation panel in mdBook.

Proposed Solution

Add a grab handle to the UI, next to the sidebar (like this, though the mockup shows rustdoc instead of mdbook).

Notes

No response