Update mdbook to 0.4.28

This version contains rust-lang/mdBook#1986 which will allow us to test the code in each translation.

Author: mgeisler

.github/workflows/install-mdbook/action.yml

@@ -8,7 +8,7 @@ runs:
     # The --locked flag is important for reproducible builds. It also
     # avoids breakage due to skews between mdbook and mdbook-svgbob.
     - name: Install mdbook
-      run: cargo install mdbook --locked --version 0.4.25
+      run: cargo install mdbook --locked --version 0.4.28
       shell: bash
 
     - name: Install mdbook-svgbob