Merge pull request #3828 from rust-lang/renovate/mdbook-0.x

Update dependency mdbook to v0.4.51

Author: ehuss

.github/workflows/deploy.yml

@@ -6,7 +6,7 @@ on:
 
 env:
   # renovate: datasource=crate depName=mdbook versioning=semver
-  MDBOOK_VERSION: 0.4.48
+  MDBOOK_VERSION: 0.4.51
 
 jobs:
   build: