Update Rust crate mdbook to ^0.4.21

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
mdbook	dependencies	patch	^0.4.18 -> ^0.4.21

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Release Notes

rust-lang/mdBook

v0.4.21

Compare Source

92afe9b...8f01d02

Fixed

• Fixed an issue where mdBook would fail to compile with Rust nightly-2022-07-22.
  #​1861

v0.4.20

Compare Source

53055e0...da166e0

Fixed

• Fixed a regression in 0.4.19 where inline code would have excessive padding
  in some situations such as headings.
  #​1855

v0.4.19

Compare Source

ae275ad...53055e0

Added

• The serve command now supports HEAD requests.
  #​1825

Changed

• An error is now generated when a custom theme directory does not exist.
  #​1791
• Very wide tables now have independent horizontal scrolling so that scrolling
  to see the rest of the table will not scroll the entire page.
  #​1617
• The buttons on code blocks are now only shown when the mouse cursor hovers
  over them (or tapped on mobile). There is also some extra spacing to reduce
  the overlap with the code.
  #​1806
• The first chapter always generates an index.html file. Previously it would
  only generate the index file for prefix chapters.
  #​1829

Fixed

• mdbook serve --open now properly handles the case if the first chapter is a draft.
  #​1714
  #​1830
• Very long words (over 80 characters) are no longer indexed to avoid a stack overflow.
  #​1833

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