Settings: merge visual diff and file tree diff settings

[stsewd]

What's the problem this feature will solve?

In the docs we are referring to these features as one, but in the settings they are separate, and they also don't have any other context. Would be great to link to the docs from there.

Describe the solution you'd like

Merge those settings. Maybe even rename the feature to something more explicit "visual diff" feels too generic, maybe something like "Pull request diff" is more explicit about how is the feature useful (similar to external versions being called pull request previews).

Additional context

https://github.com/readthedocs/meta/issues/176#issuecomment-2843318910