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- Removed outdated Mintlify configuration files and documentation.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/components/api-page.tsx

@@ -0,0 +1,7 @@
+import { openapi } from '@/lib/openapi';
+import { createAPIPage } from 'fumadocs-openapi/ui';
+import client from './api-page.client';
+
+export const APIPage = createAPIPage(openapi, {
+  client,
+});