docs(snapshots): Add wipe and onion diff mode examples (#17969)

## DESCRIBE YOUR PR

Improves the "Sentry UI" section of the reviewing-snapshots guide by
giving snapshot diff modes their own "Viewing Snapshot Diffs" subsection
with example images for all three modes.

- Add example images for **wipe** and **onion** diff modes (previously
only **split** had one)
- Note that the split mode mask overlay color can be changed or hidden
- Restructure the diff mode descriptions for readability

## IS YOUR CHANGE URGENT?

Help us prioritize incoming PRs by letting us know when the change needs
to go live.
- [ ] Urgent deadline (GA date, etc.): <!-- ENTER DATE HERE -->
- [ ] Other deadline: <!-- ENTER DATE HERE -->
- [x] None: Not urgent, can wait up to 1 week+

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've
added an urgent due date to it.
Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: mtopo27

docs/product/snapshots/img/sentry_ui_onion_diff.png

@@ -56,14 +56,24 @@ In your project settings, you can configure status check behavior to conditional
 
 ## Sentry UI
 
-You can view all images for an upload in the Sentry UI.You can filter for specific statuses (modified, added, removed, renamed, unchanged) and view the diff in three modes: split, wipe, and onion:
+You can view all images for an upload in the Sentry UI and can filter for specific statuses (modified, added, removed, renamed, unchanged).
 
-- **Split** — Side-by-side view of base and current branch with a diff overlay highlighting changed pixels.
-- **Wipe** — Drag a slider across the image to compare base and current branch.
-- **Onion** — Overlay both images with an adjustable opacity slider.
+### Viewing Snapshot Diffs
+
+There are three diff modes:
+
+**Split:** Side-by-side view of base and current branch with a diff overlay highlighting changed pixels. Optionally you can choose to change the color or hide the mask overlay.
 
 ![Example of the Sentry UI split diff mode](../img/sentry_ui_split_diff.png)
 
+**Wipe:** Drag a slider across the image to compare base and current branch.
+
+![Example of the Sentry UI wipe diff mode](../img/sentry_ui_wipe_diff.png)
+
+**Onion:** Overlay both images with an adjustable opacity slider.
+
+![Example of the Sentry UI onion diff mode](../img/sentry_ui_onion_diff.png)
+
 ### Finding Snapshots
 
 You can get to a specific snapshot from the links on the status check or through the Sentry Releases page.