Skip mdbook install in CI when already cached

Use 'which' to check if the binary exists before running
cargo install, avoiding both install failures and unnecessary
recompilation on cache hits.

Author: atulkhare4096

.github/workflows/pages.yml

@@ -42,8 +42,8 @@ jobs:
 
       - name: Install mdbook and dependencies
         run: |
-          cargo install mdbook
-          cargo install mdbook-mermaid
+          which mdbook || cargo install mdbook
+          which mdbook-mermaid || cargo install mdbook-mermaid
 
       - name: Build documentation
         run: cargo xtask deploy