Rework diff docs

[stsewd]

• Visual diff is kind of limited to highlighting the differences in the page itself (we already got a user confused about this). I'm suggesting something broader, pull request diff
• Include more things as limitations
• Regex aren't supported for ignoring files, what we support are some basic patterns.

I also think we should move the configuration of the feature outside the addon, the addon should only control if the diff menu is visible in the preview or not. The settings like ignored files, base version, if a comment should be posted in the PR should be in the PR form.

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--12276.org.readthedocs.build/12276/

• Developer's documentation (dev): https://dev--12276.org.readthedocs.build/12276/

[stsewd]

@ericholscher let me know what you think, can also try porting some of the improvements over the existing docs without changing the names and structure.

[ericholscher]

@stsewd I think in the future we might want to expand this to diffing between active versions, so I don't think we want to brand this PR diff? We have already talked extensively about branding here, I'd be very hesitant to change it again without a larger discussion.

Overall the copy changes look good though, but we should separate improvements from rebranding discussions, which should happen in an issue or team call, and not a PR.