Add GitHub workflow config to add Vale doc linting.

.github/workflows/vale-doc-lint.yml

@@ -0,0 +1,25 @@
+name: Check Docusaurus docs with Vale linter
+
+on: [pull_request]
+
+jobs:
+  vale:
+    name: Vale doc linter
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: errata-ai/vale-action@reviewdog
+        with:
+          # added, diff_context, file, nofilter
+          # Default is added: results are filtered for added/modified files. Set to no filter when all files need to be checked.
+          # More info: https://github.com/errata-ai/vale-action and https://github.com/reviewdog/reviewdog#filter-mode
+          # filter_mode: nofilter
+          # github-pr-check, github-pr-review, github-check
+          reporter: github-pr-check
+          # Set fail_on_error to true to make sure builds fail.
+          fail_on_error: true
+          files: docusaurus
+        env:
+          # Required, set by GitHub actions automatically:
+          # https://docs.github.com/en/actions/security-guides/automatic-token-authentication#about-the-github_token-secret
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}