actions: Link check

#19

.github/workflows/linkcheck.yml

@@ -0,0 +1,20 @@
+name: Link Check
+on: [push, pull_request]
+
+jobs:
+  linkcheck:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup python
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
+        architecture: x64
+    - uses: actions/cache@v1
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+    - run: pip install -r requirements.txt
+    - run: cd ./docs/ && make dirhtml
+    - run: cd ./docs/ && make linkcheck