Fix github workflow by adding gem dependancies

Signed-off-by: Atish Patra <atishp@rivosinc.com>

.github/workflows/build-doc.yml

@@ -37,6 +37,14 @@ jobs:
         sudo apt-get update
         sudo apt-get install -y pandoc asciidoctor ditaa ruby-asciidoctor-pdf
 
+    - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.0.6' # specify the Ruby version you need
+
+      - name: Install asciidoctor-bibtex gem
+        run: gem install asciidoctor-bibtex
+
     # Step 3: Build Files
     - name: Build Files
       run: make