[workflow] Use Sphinx problem matcher on GitHub docs builds (GH-20325)

.github/problem-matchers/sphinx.json

@@ -0,0 +1,40 @@
+{
+    "problemMatcher": [
+        {
+            "owner": "sphinx-problem-matcher",
+            "pattern": [
+                {
+                    "regexp": "^(.*):(\\d+):\\s+(\\w*):\\s+(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "severity": 3,
+                    "message": 4
+                }
+            ]
+        },
+        {
+            "owner": "sphinx-problem-matcher-loose",
+            "pattern": [
+                {
+                    "_comment": "A bit of a looser pattern, doesn't look for line numbers, just looks for file names relying on them to start with / and end with .rst",
+                    "regexp": "(\/.*\\.rst):\\s+(\\w*):\\s+(.*)$",
+                    "file": 1,
+                    "severity": 2,
+                    "message": 3
+                }
+            ]
+        },
+        {
+            "owner": "sphinx-problem-matcher-loose-no-severity",
+            "pattern": [
+                {
+                    "_comment": "Looks for file names ending with .rst and line numbers but without severity",
+                    "regexp": "^(.*\\.rst):(\\d+):(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "message": 3
+                }
+            ]           
+        }
+    ]
+}

.github/workflows/doc.yml

@@ -25,6 +25,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+    - name: Register Sphinx problem matcher
+      run: echo "::add-matcher::.github/problem-matchers/sphinx.json"
     - name: 'Install Dependencies'
       run: sudo ./.github/workflows/posix-deps-apt.sh && sudo apt-get install wamerican
     - name: 'Configure CPython'
@@ -34,7 +36,7 @@ jobs:
     - name: 'Install build dependencies'
       run: make -C Doc/ PYTHON=../python venv
     - name: 'Build documentation'
-      run: xvfb-run make -C Doc/ PYTHON=../python SPHINXOPTS="-q -W -j4" doctest suspicious html
+      run: xvfb-run make -C Doc/ PYTHON=../python SPHINXOPTS="-q -W --keep-going -j4" doctest suspicious html
     - name: 'Upload'
       uses: actions/upload-artifact@v1
       with: