User-facing docs for Rapid Diffs in MRs

When we ship the first iteration of Rapid Diffs in MRs (gitlab-org#18457), there will be a toggle for user to opt-in to the experience.

We need to document this in user-facing docs:

• That the toggle exists and what it does
• How to give feedback
• What the feature gaps are (there will not be feature parity in this iteration)