text: Title: "Add page" on PageListingViewSet should skip choosing parent page if only one choice exists
  Body: ### Is your proposal related to a problem?
  
  According to [this Stack Overflow question](https://stackoverflow.com/questions/78513499/wagtail-6-1-pagelistingviewset-how-to-automatically-select-a-parent-page-when-c), the "Add new page" button on a `PageListingViewSet` view takes users to the "choose parent page" view and prompts them to select a parent page for the new page, even when only one valid parent exists.
  
  ### Describe the solution you'd like
  
  If only one valid parent page exists, the view should skip the "choose parent page" prompt, and redirect directly to the main "create page" form to create a page under that parent. This matches the old behaviour of the ModelAdmin module (in Wagtail until 5.x and now available as the wagtail-modeladmin package).
  
  ### Working on this
  
  Anyone can contribute to this. View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.
  