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- Removed outdated Mintlify configuration files and documentation.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/.gitignore

@@ -1,3 +1,26 @@
-node_modules
-.mintlify
+# deps
+/node_modules
+
+# generated content
+.source
+
+# test & build
+/coverage
+/.next/
+/out/
+/build
+*.tsbuildinfo
+
+# misc
 .DS_Store
+*.pem
+/.pnp
+.pnp.js
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# others
+.env*.local
+.vercel
+next-env.d.ts