Update documentation to show send UX without "include selection" or "replace selection" checkboxes

[JasonWeill]

Problem

#905 removed the "Include selection" checkbox from the chat UI in favor of a dropdown beside the "send" button. However, the documentation still refers to the "include selection" and "replace selection" checkboxes: https://jupyter-ai.readthedocs.io/en/latest/users/index.html#asking-about-something-in-your-notebook

Proposed Solution

Update the documentation's text and images to reflect the current message-sending UI.