Add menu_placeholder parameter to allow specifying placeholder text for the PageGroup menu

[krittick]

Summary

This adds a new parameter menu_placeholder to the Paginator class, which allows users to specify the text shown in the PaginatorMenu instance used with page groups. Previously this required subclassing Paginator and overriding add_menu() to accomplish.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)