Update Rust crate mdbook to ^0.4.31

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
mdbook	dependencies	patch	^0.4.30 -> ^0.4.31

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Release Notes

rust-lang/mdBook (mdbook)

### [`v0.4.31`](https://togithub.com/rust-lang/mdBook/blob/HEAD/CHANGELOG.md#mdBook-0431) [Compare Source](https://togithub.com/rust-lang/mdBook/compare/v0.4.30...v0.4.31) [v0.4.30...v0.4.31](https://togithub.com/rust-lang/mdBook/compare/v0.4.30...v0.4.31) ##### Fixed - Fixed menu border render flash during page navigation. [#​2101](https://togithub.com/rust-lang/mdBook/pull/2101) - Fixed flicker setting sidebar scroll position. [#​2104](https://togithub.com/rust-lang/mdBook/pull/2104) - Fixed compile error with proc-macro2 on latest Rust nightly. [#​2109](https://togithub.com/rust-lang/mdBook/pull/2109)

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