Emit mdBook version into build log

Allows confirming that we've picked up the fix for https://blog.rust-lang.org/2021/01/04/mdbook-security-advisory.html

Author: dtolnay

.github/workflows/site.yml

@@ -20,6 +20,7 @@ jobs:
           export MDBOOK_TARBALL="mdbook-${MDBOOK_VERSION}-x86_64-unknown-linux-gnu.tar.gz"
           export MDBOOK_URL="https://github.com/dtolnay/mdBook/releases/download/cxx/${MDBOOK_TARBALL}"
           curl -Lf "${MDBOOK_URL}" | tar -xzC book
+          book/mdbook --version
 
       - name: Build
         run: book/build.sh