Define the aria-label for the extra options menu of the copy page dropdown

Hi! 👋

Currently, in the copy page dropdown at the top of each page on the documentation pages, there is no clear label for the menu next to the "Copy page" (as Markdown) button (a screen reader only announces that it is a menu). That being said, I think it would be useful to add an aria-label to this menu to make it clearer. Two suggestions:

• More AI options
• More Markdown options (source)

What do you think? Let me know if I can open a PR if you agree with the change.

Steps to Reproduce

1. Go to https://tanstack.com/start/latest/docs/framework/react/quick-start (or a similar documentation page).
2. Turn on the screen reader (for example, VoiceOver on macOS) and tab to the "Copy page" button.
3. Press tab again and listen to the "generic" announcement about this element being a menu.

src/components/CopyPageDropdown.tsx の 305〜314 行付近から始め、リンク先のクイックスタートページのようなドキュメントページで、スクリーンリーダーを使ってメニューの読み上げを再現します。追加オプションメニューに明確な aria-label を追加し、メニューが単に汎用的なメニューとしてではなく、意味のある形で読み上げられることを確認します。