Update documentation website to `tailwind` `v4`

[roynakakawa]

Why

Tailwind CSS latest stable version v4.3 is out.

What

Upgrade to latest version after completing #3403

How

• Update TailwindCSS to v4.3.
• Add PostCSS.
• Check in docusaurus.config.js that tailwind is correctly imported.
• Update custom.css with correct Tailwind imports (using @layer).
• Check if preflight needs wrapping.
• Check for conflicts in Infima vs Tailwind.
• Check and fix if styling issues appears.