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/core/data-attributes.md

@@ -137,7 +137,7 @@ Text modifier. Supports multiple space-separated values.
 
 ## data-layout
 
-Layout pattern. See [Layouts](/docs/layouts/) for the full reference.
+Layout pattern. See [Layouts](/docs/components/layouts/) for the full reference.
 
 Common values: `docs`, `hero`, `install`, `actions`, `grid-2`, `grid-3`, `card-grid`, `stats`, `prose`, `fluid`, `timeline`, `report`.
 
@@ -155,8 +155,8 @@ Navigation variant. Applied to `<nav>` elements.
 ```html
 <nav data-nav="sidebar" aria-label="Documentation">
   <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>
 ```
@@ -357,6 +357,6 @@ Document metadata patterns for structured article content.
 
 ## Related
 
-- [Layouts](/docs/layouts/) — `data-layout` values in full
-- [Semantic HTML](/docs/semantic-html/) — how plain elements are styled without attributes
+- [Layouts](/docs/components/layouts/) — `data-layout` values in full
+- [Semantic HTML](/docs/core/semantic-html/) — how plain elements are styled without attributes
 - [Components](/docs/components/) — callouts, tasks, and session blocks in detail