Update Rust crate mdbook to ^0.4.28

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
mdbook	dependencies	patch	^0.4.26 -> ^0.4.28

---

Release Notes

rust-lang/mdBook

### [`v0.4.28`](https://togithub.com/rust-lang/mdBook/blob/HEAD/CHANGELOG.md#mdBook-0428) [Compare Source](https://togithub.com/rust-lang/mdBook/compare/v0.4.27...v0.4.28) [v0.4.27...v0.4.28](https://togithub.com/rust-lang/mdBook/compare/v0.4.27...v0.4.28) ##### Changed - The sidebar is now shown on wide screens when localstorage is disabled. [#​2017](https://togithub.com/rust-lang/mdBook/pull/2017) - Preprocessors are now run with `mdbook test`. [#​1986](https://togithub.com/rust-lang/mdBook/pull/1986) ##### Fixed - Fixed regression in 0.4.26 that prevented the title bar from scrolling properly on smaller screens. [#​2039](https://togithub.com/rust-lang/mdBook/pull/2039)

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• [ ] If you want to rebase/retry this PR, check this box

---

This PR has been generated by Mend Renovate. View repository job log here.