Update Rust crate mdbook to ^0.4.29

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
mdbook	dependencies	patch	^0.4.28 -> ^0.4.29

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Release Notes

rust-lang/mdBook

### [`v0.4.29`](https://togithub.com/rust-lang/mdBook/blob/HEAD/CHANGELOG.md#mdBook-0429) [Compare Source](https://togithub.com/rust-lang/mdBook/compare/v0.4.28...v0.4.29) [v0.4.28...v0.4.29](https://togithub.com/rust-lang/mdBook/compare/v0.4.28...v0.4.29) ##### Changed - Built-in fonts are no longer copied when `fonts/fonts.css` is overridden in the theme directory. Additionally, the warning about `copy-fonts` has been removed if `fonts/fonts.css` is specified. [#​2080](https://togithub.com/rust-lang/mdBook/pull/2080) - `mdbook init --force` now skips all interactive prompts as intended. [#​2057](https://togithub.com/rust-lang/mdBook/pull/2057) - Updated dependencies [#​2063](https://togithub.com/rust-lang/mdBook/pull/2063) [#​2086](https://togithub.com/rust-lang/mdBook/pull/2086) [#​2082](https://togithub.com/rust-lang/mdBook/pull/2082) [#​2084](https://togithub.com/rust-lang/mdBook/pull/2084) [#​2085](https://togithub.com/rust-lang/mdBook/pull/2085) ##### Fixed - Switched from the `gitignore` library to `ignore`. This should bring some improvements with gitignore handling. [#​2076](https://togithub.com/rust-lang/mdBook/pull/2076)

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