add ci to build documentation

.github/workflows/mkdoc.yml

@@ -0,0 +1,22 @@
+name: mkdocs
+on:
+  push:
+    paths:
+    - 'docs/**'
+
+jobs:
+  linux:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.8
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.8
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install mkdocs
+    - name: Build doc
+      run: |
+        mkdocs gh-deploy