Refactor documentation structure and dependencies for migration to Fumadocs

- Updated `.gitignore` to include new build and generated content directories.
- Removed outdated Mintlify configuration files and documentation.
- Introduced new `MIGRATION.md` to outline the transition from Mintlify to Fumadocs.
- Added `mdx-components.tsx` for MDX component configuration and compatibility.
- Updated `package.json` and `next.config.mjs` for new dependencies and Next.js configuration.
- Created `source.config.ts` for content source configuration.
- Added OpenAPI specification in `openapi.json` for API documentation.
- Removed legacy files and adjusted project structure to align with Fumadocs conventions.

docs/app/(home)/layout.tsx

@@ -0,0 +1,6 @@
+import { HomeLayout } from 'fumadocs-ui/layouts/home';
+import { baseOptions } from '@/lib/layout.shared';
+
+export default function Layout({ children }: LayoutProps<'/'>) {
+  return <HomeLayout {...baseOptions()}>{children}</HomeLayout>;
+}

docs/app/(home)/page.tsx

@@ -0,0 +1,16 @@
+import Link from 'next/link';
+
+export default function HomePage() {
+  return (
+    <div className="flex flex-col justify-center text-center flex-1">
+      <h1 className="text-2xl font-bold mb-4">Hello World</h1>
+      <p>
+        You can open{' '}
+        <Link href="/docs" className="font-medium underline">
+          /docs
+        </Link>{' '}
+        and see the documentation.
+      </p>
+    </div>
+  );
+}