travis: added spelling check workflow. (#811)

.github/workflows/markdownlint.yml

@@ -0,0 +1,18 @@
+name: Linting on markdown files
+on:
+  pull_request_target:
+  push:
+
+jobs:
+  lint-content:
+    name: Lint content markdown files
+    runs-on: ubuntu-latest
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v2
+
+    - name: Lint markdown files
+      uses: avto-dev/markdown-lint@v1
+      with:
+        config: './.markdownlint.jsonc'
+        args: './content/**/*.md'