Update sphinx_build_deploy.yml

First pass at trying to add caching for dependencies.

.github/workflows/sphinx_build_deploy.yml

@@ -18,10 +18,25 @@ jobs:
         with:
           python-version: '3.8'
 
+      - name: Install pip
+        run: |
+          python -m pip install --upgrade pip
+     
+      - name: Get pip cache dir
+        id: pip-cache
+        run : echo "::set-output name=dir::$(pip cache dir)"
+          
+      - name: Cache dependencies
+        uses: actions/cache@v1
+        with:
+          path: ${{ steps.pip-cache.outputs.dir }}
+          key: ${{ runner.os }}-pip-${{ hashFiles{'docs/requirements.txt') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-   
+      
       - name: Install dependencies
         run: |
           sudo apt install pandoc
-          python -m pip install --upgrade pip
           pip install -r docs/requirements.txt
           
       - name: Install ReadTheDocs theme