ci: add make linkcheck

Problem: Broken links in docs are not detected in ci.

Add a make linkcheck action to the ci.

Author: grondo

.github/workflows/main.yml

@@ -1,4 +1,4 @@
-on: [ pull_request ]
+on: [ pull_request, push ]
 jobs:
   check-pr:
     name: validate commits
@@ -27,3 +27,20 @@ jobs:
     - name: make check
       run: |
         make check
+
+  make-linkcheck:
+    name: make linkcheck
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
+    - uses: actions/setup-python@v1
+      with:
+        python-version: 3.6
+    - name: install dependencies
+      run: |
+        pip3 install --upgrade -r ./requirements.txt
+    - name: make linkcheck
+      run: |
+        make linkcheck