Add mobile performance score section (#14923)

## DESCRIBE YOUR PR
Adds a "Performance Score" section to the Mobile Vitals documentation
(`/product/insights/mobile/mobile-vitals/`) with a table detailing the
thresholds for "Good", "Meh", and "Poor" average cold and warm app start
times. This change addresses a user request to clarify how these mobile
vital metrics are scored within Sentry, similar to the existing Web
Vitals performance score documentation.

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Co-authored-by: Cursor Agent <[email protected]>

Author: sergical

docs/product/insights/mobile/mobile-vitals/index.mdx

@@ -32,6 +32,15 @@ In the example below, the detail view of a transaction displays the warm start m
 
 While the SDKs differentiate between cold and warm starts, they don't track hot starts or resumes. To get more insight into your cold and warm start metrics, you can use the [App Starts](/product/insights/mobile/mobile-vitals/app-starts/) feature.
 
+## Performance Score
+
+Sentry categorizes average app start durations as follows:
+
+| Metric | Good | Meh | Poor |
+| --- | --- | --- | --- |
+| Average Cold App Start | less than 3s | between 3s and 5s | greater than 5s |
+| Average Warm App Start | less than 1s | between 1s and 2s | greater than 2s |
+
 ## Slow and Frozen Frames
 
 To track the responsiveness of the user interface, Sentry measures _slow frames_ and _frozen frames_. Both indicate if the rendering of a frame is taking too long, causing the UI to appear laggy or frozen.