docs: migrate Jekyll site to Hugo + Hextra

Replace docs/_config.yml + docs/SUMMARY.md with Hugo + Hextra theme.
Move all content into docs/content/, images into docs/static/images/.
Update docs_consistency_test.go to check Hugo front matter instead of
SUMMARY.md inclusion. Update CI workflow and screenshot script paths.

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

.github/workflows/docs.yml

@@ -23,11 +23,15 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - name: Setup Pages
         uses: actions/configure-pages@45bfe0192ca1faeb007ade9deae92b16b8254a0d # v6.0.0
-      - name: Build with Jekyll
-        uses: actions/jekyll-build-pages@44a6e6beabd48582f863aeeb6cb2151cc1716697 # v1.0.13
+      - name: Setup Hugo
+        uses: peaceiris/actions-hugo@75d2a84ef14cf28a37009d2a8f1d0d6e67eb56e8 # v3.0.0
         with:
-          source: 'docs/'
-          destination: '_site'
+          hugo-version: 'latest'
+          extended: true
+      - name: Build with Hugo
+        run: hugo --source docs/ --minify --destination ../_site
+        env:
+          HUGO_ENVIRONMENT: production
       - name: Upload artifact
         uses: actions/upload-pages-artifact@fc324d3547104276b827a68afc52ff2a11cc49c9 # v5.0.0
         with: