CI: Pin mdbook to 0.4.43

This should prevent future surprising changes to the site from newly
published mdbook versions with breaking changes. For example mdbook
0.4.41 introduced some changes that needed the `index.hbs` file to be
updated. See the parent commit.

.github/workflows/gh-pages.yml

@@ -16,8 +16,8 @@ jobs:
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v2
         with:
-          mdbook-version: 'latest'
-          # mdbook-version: '0.4.8'
+          # mdbook-version: 'latest'
+          mdbook-version: '0.4.43'
 
       - run: mdbook build book