chore: bump docs dependencies to latest

Upgrade the docs site to Next 16 and React 19:

- next 15.5.9 -> 16.3.0
- react / react-dom 18.2.0 -> 19.2.8
- @vercel/analytics 1.1.1 -> 2.0.1
- docsgpt-react 0.5.1 -> 0.6.3
- pagefind 1.4.0 -> 1.5.2
- typescript 5.9.3 -> 6.0.3

Two deliberate holds:

Next 16 makes Turbopack the default builder, but nextra 4.6.1 is not
compatible with it. Its loader calls transformerTwoslash() at module top
level, and twoslash gets an undefined ts.sys inside Turbopack's
webpack-loader shim, so every .mdx file fails to compile. Nextra offers no
option to disable twoslash, so the dev and build scripts opt out with
--webpack until nextra supports Turbopack.

typescript stays on 6.0.3 rather than 7.0.2 for the same reason: TS 7 is the
native rewrite and drops the ts.sys API that @typescript/vfs and twoslash
rely on, which breaks the build even under webpack. twoslash declares
typescript ^5.5.0 || ^6.0.0, so 6.0.3 is the ceiling this toolchain supports.

nextra and nextra-theme-docs were already at the latest 4.6.1.

docs/package.json

@@ -1,22 +1,22 @@
 {
   "scripts": {
-    "dev": "next dev",
-    "build": "next build",
+    "dev": "next dev --webpack",
+    "build": "next build --webpack",
     "postbuild": "pagefind --site .next/server/app --output-path public/_pagefind",
     "start": "next start"
   },
   "license": "MIT",
   "dependencies": {
-    "@vercel/analytics": "^1.1.1",
-    "docsgpt-react": "^0.5.1",
-    "next": "^15.5.15",
+    "@vercel/analytics": "^2.0.1",
+    "docsgpt-react": "^0.6.3",
+    "next": "^16.3.0",
     "nextra": "^4.6.1",
     "nextra-theme-docs": "^4.6.1",
-    "react": "^18.2.0",
-    "react-dom": "^18.2.0"
+    "react": "^19.2.8",
+    "react-dom": "^19.2.8"
   },
   "devDependencies": {
-    "pagefind": "^1.3.0",
-    "typescript": "^5.9.3"
+    "pagefind": "^1.5.2",
+    "typescript": "^6.0.3"
   }
 }