Revert "build(deps): bump next from 15.5.2 to 16.1.5 in /documentation (#347)" (#349)

This reverts commit ea6c521.

This is because upgrading breaks the doc website as Nextra 3.x not
compatible with Nextjs 16.x

Author: qbarbe-dashlane

documentation/package.json

@@ -3,7 +3,7 @@
     "type": "module",
     "packageManager": "yarn@4.5.1",
     "dependencies": {
-        "next": "^16.1.5",
+        "next": "^15.4.7",
         "nextra": "^3.2.3",
         "nextra-theme-docs": "^3.2.3",
         "react": "^18.3.1",