[Docs] Update references from "Apache Arrow DataFusion" to "Apache DataFusion"

[Hyunseok Seo]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Need to update DataFusion references post-promotion to top-level project.

Describe the solution you'd like

Change all "Apache Arrow DataFusion" mentions to "Apache DataFusion."

Describe alternatives you've considered

Alternative is to keep the current references, risking confusion.

Additional context

Updating will clarify project structure for new users and contributors.