Install mkdocs for ci

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Set up Python
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
 
@@ -125,7 +125,7 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Set up Python
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
 
@@ -184,9 +184,15 @@ jobs:
       run: |
         ./tests/test_docs.sh
 
+    - name: Set up Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
+
     - name: Documentation build
       run: |
         pip install -r docs/requirements-docs.txt
+        pip install mkdocs
         mkdocs build
 
     - name: Slack Notification
@@ -229,7 +235,7 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Set up Python
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v2
       with:
         python-version: 3.8