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.

Inizia in src/components/CopyPageDropdown.tsx, intorno alle righe 305-314, e riproduci l’annuncio del menu con uno screen reader in una pagina di documentazione come la pagina di avvio rapido collegata. Aggiungi un aria-label chiaro al menu delle opzioni aggiuntive, quindi verifica che il menu venga annunciato in modo significativo invece che soltanto come un menu generico.