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.

Commencez dans src/components/CopyPageDropdown.tsx, autour des lignes 305-314, et reproduisez l’annonce du menu avec un lecteur d’écran sur une page de documentation telle que la page de démarrage rapide liée. Ajoutez un aria-label explicite au menu d’options supplémentaires, puis vérifiez que le menu est annoncé de manière significative plutôt que seulement comme un menu générique.