Add sphinx problem matcher

.github/sphinx-problem-matcher.json

@@ -0,0 +1,18 @@
+{
+    "problemMatcher": [
+        {
+            "owner": "sphinx",
+            "pattern": [
+                {
+                    "regexp": "^Warning, treated as error:$"
+                },
+                {
+                    "regexp": "^(.*?):(\\d+):(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "message": 3
+                }
+            ]
+        }
+    ]
+}

.github/workflows/ci.yaml

@@ -59,6 +59,9 @@ jobs:
           python -m pip install --upgrade pip
           python -m pip install tox sphinx
 
+      - name: Add problem matcher
+        run: echo "::add-matcher::.github/sphinx-problem-matcher.json"
+
       - name: Build and check docs using tox
         run: tox -e docs