docs: integrate Vale for documentation linting (#2242)

* added value rules

* resolved errors raised by vale

* added github action

* fixes to github action

* added details in contribution guidelines

* added rule for numbers

* limited checks to errors

.github/workflows/docs-test.yml

@@ -1,4 +1,4 @@
-name: Docusaurus Documentation Test
+name: Documentation Tests
 on: 
   pull_request:
     paths:
@@ -34,3 +34,29 @@ jobs:
           cd www/docs
           yarn install
           yarn build
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Cancel Previous Runs
+        uses: styfle/cancel-workflow-action@0.9.1
+        with:
+          access_token: ${{ github.token }}
+
+      - name: Checkout
+        uses: actions/checkout@v2.3.5
+        with:
+          fetch-depth: 0
+
+      - name: Get Directories to Scan
+        working-directory: docs
+        run: ./get-files.sh
+        id: directories
+
+      - name: Vale Linter
+        uses: errata-ai/vale-action@reviewdog
+        with:
+          files: ${{ steps.directories.outputs.LIST }}
+          fail_on_error: true
+          vale_flags: '--minAlertLevel=error'
+        env:
+          GITHUB_TOKEN: ${{ github.token }}