Clarify behaviour of sort order when no siblings

In [new card sort order](https://docs.ankiweb.net/deck-options.html#new-card-sort-order), almost every option is explained in terms of what happens to siblings. This caused a lot of confusion when I first switched over to v3 as I had no siblings.

I suggest we also describe the other possible situation and what will happen in those cases when there are no siblings. Standard practice in technical writing afaik.

Start with the linked “new card sort order” section in the Anki documentation and review how each option is currently explained in terms of siblings. Confirm and document the behavior for cards with no siblings, so the section covers both situations clearly.

Written by the indexing model from the issue text.