upgrade to latest Fumadocs, Next.js 15.3.1, and Tailwind v4 #1000
Description
update the documentation site to use the latest versions of core dependencies including Fumadocs, Next.js 15.3.1 with Turbopack, and Tailwind CSS v4.
- upgrade Fumadocs to latest version
- migrate to Next.js 15.3.1 with Turbopack
- upgrade and migrate to Tailwind CSS v4
- update build configuration for Turbopack
- fix breaking changes from dependency updates