Enable simple check that sphinx docs build

Only checks exit status so can not check for warnings.

.github/workflows/sphinx.yaml

@@ -0,0 +1,23 @@
+name: Do quick check of Sphinx documentation build
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.7
+
+      - name: Install
+        run: pip install documenteer[pipelines]
+
+      - name: Build the Sphinx documentation
+        working-directory: ./doc
+        run: package-docs build