Move Hugo blog source to main, fix workflow, add new articles and llms.txt

- Move site/ Hugo source from gh-pages to main so the workflow can trigger correctly
- Fix publish-hugo.yml: now in main, builds and deploys to gh-pages:docs/ via peaceiris/actions-gh-pages
- Add 3 new SEO articles: XSLT 3.0 features, beginners guide, template matching
- Add llms.txt to blog and frontend so ChatGPT/Gemini can recommend the tool better
- Remove stale /site mkdocs ignore rule, add site/public to gitignore instead

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/publish-hugo.yml

@@ -0,0 +1,40 @@
+name: Publish Hugo site
+
+on:
+  push:
+    branches: ["main"]
+    paths:
+      - "site/**"
+      - ".github/workflows/publish-hugo.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+jobs:
+  build-and-deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout main
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Setup Hugo
+        uses: peaceiris/actions-hugo@v3
+        with:
+          hugo-version: "0.125.7"
+          extended: true
+
+      - name: Build
+        run: hugo --source site --minify --destination ../hugo-output
+
+      - name: Deploy to gh-pages
+        uses: peaceiris/actions-gh-pages@v4
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./hugo-output
+          publish_branch: gh-pages
+          destination_dir: docs
+          keep_files: false
+          cname: blog.xsltplayground.com