ci: install mpi for docs workflow

Author: cwsmith

.github/workflows/doxygen-gh-pages.yml

@@ -22,6 +22,10 @@ jobs:
               sudo apt update
               sudo apt install g++ cmake doxygen
 
+      - name: Install mpi
+        run: |
+             sudo apt-get install mpich libmpich-dev
+
       - name: Kokkos Checkout repo
         uses: actions/checkout@v4
         with:
@@ -88,4 +92,4 @@ jobs:
 
       - name: Deploy to GitHub Pages
         id: deployment
-        uses: actions/deploy-pages@v4
+        uses: actions/deploy-pages@v4