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>

site/layouts/_default/single.html

@@ -0,0 +1,13 @@
+{{ define "main" }}
+<article class="post">
+  <p class="eyebrow">{{ .Section | title }}</p>
+  <h1>{{ .Title }}</h1>
+  <p class="muted">{{ .Date.Format "02 Jan 2006" }}</p>
+  {{ with .Params.description }}<p class="lead">{{ . }}</p>{{ end }}
+  {{ partial "adsense/in-article.html" . }}
+  <div class="content">
+    {{ .Content }}
+  </div>
+  <p class="muted back-link"><a href="{{ .CurrentSection.RelPermalink }}">← Back to {{ .CurrentSection.Title }}</a></p>
+</article>
+{{ end }}