Fix build error: move markdown.tsx out of pages directory

The file src/pages/changelog/markdown.tsx was being treated as a page
component by Docusaurus but lacked a default export, causing SSG to fail.

Moved it to src/components/changelog/ReleaseBody.tsx and updated the
import in index.tsx to fix the build error.

Author: Copilot

src/pages/changelog/markdown.tsx src/components/changelog/ReleaseBody.tsxsrc/pages/changelog/markdown.tsx renamed to src/components/changelog/ReleaseBody.tsx

@@ -1,7 +1,7 @@
 import React, { useState } from "react";
 import Layout from "@theme/Layout";
 import releases from "@site/src/data/releases.json";
-import { ReleaseBody } from "./markdown";
+import { ReleaseBody } from "@site/src/components/changelog/ReleaseBody";
 import "./index.css";
 
 type Release = {