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>

site/layouts/docs/single.html

@@ -15,46 +15,19 @@
   <section data-layout="docs">
 
     <nav aria-label="Documentation" data-nav="sidebar">
-      {{- $menu := index .Site.Menus "docs" -}}
-      {{- if $menu -}}
-        {{- range $menu -}}
-          {{- if .HasChildren -}}
-          <h3>{{ .Name }}</h3>
-          <ul>
-            {{- range .Children -}}
-            <li>
-              <a href="{{ .URL }}"
-                {{- if eq (relURL .URL) $.RelPermalink }} aria-current="page"{{ end -}}>
-                {{- .Name -}}
-              </a>
-            </li>
-            {{- end -}}
-          </ul>
-          {{- else -}}
-          <ul>
-            <li>
-              <a href="{{ .URL }}"
-                {{- if eq (relURL .URL) $.RelPermalink }} aria-current="page"{{ end -}}>
-                {{- .Name -}}
-              </a>
-            </li>
-          </ul>
-          {{- end -}}
-        {{- end -}}
-      {{- else -}}
-        {{- with .CurrentSection -}}
-        <h3>{{ .Title }}</h3>
-        <ul>
-          {{- range .RegularPages -}}
-          <li>
-            <a href="{{ .RelPermalink }}"
-               {{- if eq .RelPermalink $.RelPermalink }} aria-current="page"{{ end -}}>
-              {{- .LinkTitle -}}
-            </a>
-          </li>
-          {{- end -}}
-        </ul>
+      {{- $top := .FirstSection -}}
+      {{- range $top.Sections.ByWeight -}}
+      <h3>{{ .Title }}</h3>
+      <ul>
+        {{- range .RegularPages.ByWeight -}}
+        <li>
+          <a href="{{ .RelPermalink }}"
+            {{- if eq .RelPermalink $.RelPermalink }} aria-current="page"{{ end -}}>
+            {{- .LinkTitle -}}
+          </a>
+        </li>
         {{- end -}}
+      </ul>
       {{- end -}}
     </nav>