add v3 docs workflow

.github/workflows/v3-docs.yml

@@ -0,0 +1,25 @@
+name: v3 docs
+on:
+  push:
+    branches:
+      - v3-alpha
+permissions:
+  contents: write
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-python@v4
+        with:
+          python-version: 3.x
+      - run: echo "cache_id=$(date --utc '+%V')" >> $GITHUB_ENV
+      - uses: actions/cache@v3
+        with:
+          key: mkdocs-material-${{ env.cache_id }}
+          path: .cache
+          restore-keys: |
+            mkdocs-material-
+      - run: pip install -r requirements.insiders.txt
+      - run: pip install git+https://${GH_TOKEN}@github.com/squidfunk/mkdocs-material-insiders.git
+      - run: mkdocs gh-deploy --force