Add htmlvalidator GHA

Author: akurtakov

.github/workflows/htmlvalidator.yml

@@ -0,0 +1,17 @@
+name: HTML Check
+
+on:
+  pull_request:
+    paths:
+      - '**.htm*'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v5 # Requried will all actions
+    - name: Checks HTML5
+      uses: Cyb3r-Jak3/html5validator-action@v8.0.0
+      with:
+        root: /github/workspace/org.eclipse.pde.doc.user
+        blacklist: about.html notices.html