Ignore '/docs' on some GH actions, give docs its own action (#10949)

* ignore docs on several actions, give docs its own action

* Random doc change to see if the new action runs.

* umm... not paths-ignore, paths!

* eof fix

* enabling typescript checking on docs

.github/workflows/superset-docs.yml

@@ -0,0 +1,30 @@
+name: Docs
+
+on:
+  push:
+    paths:
+      - 'docs/**'
+  pull_request:
+    paths:
+      - 'docs/**'
+
+jobs:
+  docs:
+    name: build
+    runs-on: ubuntu-18.04
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Install dependencies
+    - name: npm install
+      working-directory: ./docs
+      run: |
+        npm install
+    - name: lint
+      working-directory: ./docs
+      run: |
+        npm run lint
+    - name: gatsby build
+      working-directory: ./docs
+      run: |
+        npm run build