add vale github action

Author: devalog

.github/workflows/vale.yml

@@ -0,0 +1,23 @@
+name: Run Vale linter
+on: 
+  pull_request:
+    paths:
+      - '**.mdx'
+      - '**.md'
+      - '.vale/**'
+      - '.vale.ini'
+
+jobs:
+  vale:
+    runs-on: ubuntu-latest
+    if: |
+      github.event.pull_request.user.login != 'fern-support' &&
+      github.event.pull_request.user.login != 'github-actions'
+    steps:
+      - uses: actions/checkout@v4
+      
+      - uses: errata-ai/vale-action@reviewdog
+        with:
+          files: fern
+        env:
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}