docs(snapshots): Document tags metadata field for snapshots (#17968)

## DESCRIBE YOUR PR

Documents the `tags` metadata field for snapshots, which lets users
attach key-value string labels to snapshots and filter or group them in
the viewer (for example by theme, viewport, or component variant).

- Add the `tags` field to the snapshot metadata reference table in the
uploading-snapshots guide
- Add an example image showing how metadata appears in the Sentry UI

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## 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)
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team](https://github.com/orgs/getsentry/teams/docs)

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

Author: mtopo27

docs/product/snapshots/img/sentry_ui_metadata.png

@@ -57,6 +57,9 @@ You can include an optional JSON file to add metadata to each image:
 | `display_name`   | string | Human-readable label shown in the comparison. viewer.                                                                                                                  |
 | `group`          | string | Groups related snapshots in the UI. sidebar.                                                                                                                           |
 | `diff_threshold` | number | Value from `0.0` to `1.0`. Sentry reports the image as changed only when the share of changed pixels is greater than this value. `0.01` ignores changes of 1% or less. |
+| `tags`           | object | Key-value string labels attached to the snapshot. Use them to filter and group snapshots in the viewer, for example by theme, viewport, or component variant.          |
+
+![Example of how metadata impacts the Sentry UI](../img/sentry_ui_metadata.png)
 
 ### Supported Formats