fix: update all internal doc links to new nested URL structure

/docs/introduction/ → /docs/getting-started/introduction/
/docs/tokens/ → /docs/core/tokens/
/docs/semantic-html/ → /docs/core/semantic-html/
/docs/reset/ → /docs/core/reset/
/docs/data-attributes/ → /docs/core/data-attributes/
/docs/layouts/ → /docs/components/layouts/

Zero 404s in docs after clean build.

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

site/content/docs/components/layouts.md

@@ -25,8 +25,8 @@ Three-column scaffold: sidebar, content, TOC. Source order: left `<aside>` → c
     <nav data-nav="sidebar" aria-label="Docs">
       <small>Getting started</small>
       <ul>
-        <li><a href="/docs/introduction/" aria-current="page">Introduction</a></li>
-        <li><a href="/docs/tokens/">Token System</a></li>
+        <li><a href="/docs/getting-started/introduction/" aria-current="page">Introduction</a></li>
+        <li><a href="/docs/core/tokens/">Token System</a></li>
       </ul>
     </nav>
   </aside>
@@ -75,7 +75,7 @@ CTA button row, centered, wraps on small screens.
 
 ```html
 <nav data-layout="actions">
-  <a href="/docs/introduction/" data-role="primary">Get started →</a>
+  <a href="/docs/getting-started/introduction/" data-role="primary">Get started →</a>
   <a href="https://github.com/..." data-role="secondary">GitHub</a>
 </nav>
 ```
@@ -243,5 +243,5 @@ Renders a `<dl>` as a two-column grid: terms on the left, definitions on the rig
 
 ## Related
 
-- [Data Attributes](/docs/data-attributes/) — full `data-*` vocabulary reference
-- [Token System](/docs/tokens/) — the custom property values that layout tokens reference
+- [Data Attributes](/docs/core/data-attributes/) — full `data-*` vocabulary reference
+- [Token System](/docs/core/tokens/) — the custom property values that layout tokens reference