feat(website): Migrate to NextJS 15 + Tailwind v4

Since NextJS 15 & Tailwind v4 have been out for a while, it makes sense to begin efforts to migrate our websites before the older versions become deprecated.

https://tailwindcss.com/docs/upgrade-guide

https://nextjs.org/docs/app/guides/upgrading/version-15

Key dates:
- NextJS versions are maintained for ~2 years post-release, meaning Next 14 will be EOL on ~October 26th 2025.