Add link checker (experimental)

.github/workflows/link-checker.yml

@@ -0,0 +1,37 @@
+name: Link Checker
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      max-parallel: 4
+      matrix:
+        python-version: [3.7]
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v1
+      with:
+        python-version: ${{ matrix.python-version }}
+
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install dactyl
+
+    - name: Check for Conflict Markers
+      run: |
+        tool/conflictmarkers.sh
+
+    - name: Build docs
+      run: |
+        echo '{"github_forkurl": "https://github.com/'"${{ github.repository }}"'", "github_branch": "'"${{ github.head_ref }}"'", "github_pr_id": "TODO", "is_pr_build": true}' > dactyl_vars.json
+        tool/build_all_langs.sh --vars dactyl_vars.json
+
+    - name: Run Dactyl Link Checker
+      run: |
+        dactyl_link_checker -q