Added pa11y-ci testing with axe using GitHub action.

.github/workflows/pa11y.yml

@@ -0,0 +1,28 @@
+name: pa11y tests
+
+on: [pull_request]
+
+jobs:
+  build:
+    name: Building site and running pa11y-ci tests
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout source.
+        uses: actions/checkout@v2
+
+      - name: Install jekyll site dependencies.
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.6
+          bundler-cache: true
+
+      - name: Install pa11y-ci dependencies.
+        run: npm install
+
+      - name: Start up jekyll server.
+        run: npm run start-detached
+
+      - name: Run pa11y-ci.
+        run: npm run pa11y-ci
+

.pa11yci

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+{
+  "defaults": {
+    "concurrency": 4,
+    "standard": "WCAG2AA",
+    "runners": ["axe"]
+  },
+  "urls": [
+    "http://localhost:4000/",
+    "http://localhost:4000/docs/",
+    "http://localhost:4000/theme/"
+  ]
+}