update workflow

.github/workflows/docs.yml

@@ -3,7 +3,7 @@ name: docs
 on: 
   push:
     paths:
-      - 'docs/'
+      - 'docs/**'
 
 jobs:
   docs:

.github/workflows/docs_test.yml

@@ -0,0 +1,29 @@
+name: docs_test
+
+on: 
+  pull_request:
+    paths:
+      - 'docs/**'
+
+jobs:
+  docs_test:
+    runs-on: ubuntu-latest
+    name: Test document for pull requests
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@master
+    
+    - name: Install sphinx and manim env
+      run: |
+        pip3 install --upgrade pip
+        sudo apt install python3-setuptools
+        pip3 install -r docs/requirements.txt
+        pip3 install -r requirements.txt
+    
+    - name: Build document with Sphinx
+      run: |
+        cd docs
+        export PATH="$PATH:/home/runner/.local/bin"
+        export SPHINXBUILD="python3 -m sphinx"
+        make html