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>

.gitignore

@@ -144,7 +144,7 @@ venv.bak/
 .ropeproject
 
 # mkdocs documentation
-/site
+# /site  (removed: site/ is the Hugo blog source)
 
 # mypy
 .mypy_cache/
@@ -198,6 +198,7 @@ cython_debug/
 # Frontend
 frontend/node_modules
 frontend/dist
+site/public
 backend/server
 backend/src/.gocache
 /config/credentials.json