Test caching

Author: smistad

.github/workflows/build-and-deploy-documentation.yaml

@@ -15,6 +15,15 @@ jobs:
         run: |
           sudo apt install bison flex # Needed by Doxygen
           pip3 install jinja2         # Needed by M.css
+      - name: Cache Doxygen and M.CSS externals
+        uses: actions/cache@v4
+        with:
+          path: |
+            ${{ github.workspace }}/build/external/doxygen/
+            ${{ github.workspace }}/build/external/mcss/
+          key: |
+            ${{ hashFiles('${{ github.workspace }}/cmake/ExternalDoxygen.cmake') }}
+            ${{ hashFiles('${{ github.workspace }}/cmake/ExternalMCSS.cmake') }}
       - name: Build docs
         run: |
           mkdir build