Output lacks bypass blocks (fails WCAG 2.4.1)

[zilac-hue]

Problem

mdBook currently does not have bypass blocks to skip past content repeated on multiple pages (e.g. the left sidebar and header) when using keyboard navigation. This fails WCAG 2.4.1. Definition below is taken from WCAG 2.4.1.

A mechanism is available to bypass blocks of content that are repeated on multiple Web pages. (Level A)

Steps

1. Go to an mdbook site (was using https://doc.rust-lang.org/stable/book/)
2. Press 'tab'
3. No bypass block is focused on

Possible Solution(s)

Both Astro Starlight and Docusaurus have bypass blocks which can be used as examples. (If you go to either of those pages and press 'tab', a "Skip to main content" button appears).

Notes

No response

Version

Was using https://doc.rust-lang.org/stable/book/ to learn and noticed this, so whatever version of mdBook that was made in (also applies to https://rust-lang.github.io/mdBook/ too)