Update mdbook to 0.4.51

Author: ehuss

.github/workflows/deploy.yml

@@ -11,13 +11,15 @@ jobs:
   ci:
     name: CI
     runs-on: ubuntu-latest
+    env:
+      MDBOOK_VERSION: 0.4.51
     permissions:
       id-token: write
       contents: write
     steps:
       - uses: actions/checkout@v3
       - name: Install mdbook
-        run: curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.4.48/mdbook-v0.4.48-x86_64-unknown-linux-gnu.tar.gz | tar -xz
+        run: curl -sSL https://github.com/rust-lang/mdBook/releases/download/v${MDBOOK_VERSION}/mdbook-v${MDBOOK_VERSION}-x86_64-unknown-linux-gnu.tar.gz | tar -xz
       - name: Check blacksmith format
         run: cargo fmt --check --manifest-path=blacksmith/Cargo.toml
       - name: Build book