Update Rust crate mdbook to ^0.4.30

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
mdbook	dependencies	patch	^0.4.29 -> ^0.4.30

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Release Notes

rust-lang/mdBook

### [`v0.4.30`](https://togithub.com/rust-lang/mdBook/blob/HEAD/CHANGELOG.md#mdBook-0430) [Compare Source](https://togithub.com/rust-lang/mdBook/compare/v0.4.29...v0.4.30) [v0.4.29...v0.4.30](https://togithub.com/rust-lang/mdBook/compare/v0.4.29...v0.4.30) ##### Added - Added support for heading attributes. Attributes are specified in curly braces just after the heading text. An HTML ID can be specified with `#` and classes with `.`. For example: `## My heading {#custom-id .class1 .class2}` [#​2013](https://togithub.com/rust-lang/mdBook/pull/2013) - Added support for hidden code lines for languages other than Rust. The `output.html.code.hidelines` table allows you to define the prefix character that will be used to hide code lines based on the language. [#​2093](https://togithub.com/rust-lang/mdBook/pull/2093) ##### Fixed - Fixed a few minor markdown rendering issues. [#​2092](https://togithub.com/rust-lang/mdBook/pull/2092)

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