feat: auto-generated docs sidebar, Decap CMS, content migration infra

Docs restructure:
- Move flat docs into section subdirs (getting-started/, core/,
  components/, reference/) with _index.md for each
- Sidebar auto-generates from content structure — no manual menu entries
- New doc pages appear automatically when created in a section

Decap CMS:
- admin/index.html + config.yml for browser-based editing
- Local mode (npx decap-server) — no OAuth needed
- Collections for all content types: docs, articles, essays, notes, pages

Hugo head.html updated for new CSS layer filenames.
decap-server added as devDependency.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -9,10 +9,11 @@
     "open-props": "^1.7.0"
   },
   "devDependencies": {
+    "cssnano": "^7.0.0",
+    "decap-server": "^3.6.0",
     "postcss": "^8.4.0",
     "postcss-cli": "^11.0.0",
-    "postcss-import": "^16.0.0",
     "postcss-custom-media": "^11.0.0",
-    "cssnano": "^7.0.0"
+    "postcss-import": "^16.0.0"
   }
 }