Update dependency mdbook to v0.5.4

[renovate]

This PR contains the following updates:

Package	Update	Change
mdbook	patch	0.5.3 → 0.5.4

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Release Notes

rust-lang/mdBook (mdbook)

v0.5.4

Compare Source

v0.5.3...v0.5.4

Added

• Added zoomable images feature.
  #​3127

Changed

• Updated dependencies.
  #​3149
  #​3152
  #​3129
  #​3130

Fixed

• Fixed layout bug with images in links.
  #​3142
• Fixed previous-chapter nav hover overlapping sidebar resize handle.
  #​3134
• Fixed missing LICENSE files in published crates.
  #​3135
• Switched recommendation to linkcheck2 fork.
  #​3132
• Fixed version in CI source install docs.
  #​3154

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