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.

从 src/components/CopyPageDropdown.tsx 的第 305-314 行附近开始，在类似链接的快速入门页面这样的文档页面上使用屏幕阅读器重现菜单播报。为额外选项菜单添加清晰的 aria-label，然后验证菜单是否以有意义的方式播报，而不是仅被播报为通用菜单。