feat(docs): migrate documentation to Next.js with Fumadocs

This commit migrates the documentation site from Astro to Next.js, leveraging Fumadocs for enhanced functionality and maintainability. The migration includes:
- New Next.js configuration and setup
- Integration of Fumadocs for documentation rendering
- Addition of new documentation assets and images
- Removal of Astro-related files and configurations

The migration aims to improve the documentation site's performance, scalability, and developer experience.

apps/docs/postcss.config.mjs

@@ -0,0 +1,5 @@
+export default {
+  plugins: {
+    '@tailwindcss/postcss': {},
+  },
+};