Write Sentry docs for standalone app start and app start sample rate APIs [React Native]

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Description

Write documentation for the new standalone app start feature and the app start sample rate API for React Native.

Scope

• Document the standalone app start behavior
• Document the app start sample rate API (if applicable)
• Explain the new standalone transaction structure (app.start.cold / app.start.warm)
• Cover migration from the previous behavior (app start data attached to navigation transaction lifecycle)
• Note hybrid SDK considerations (if applicable)

Blocked by

• RN-541: Decouple app start data from navigation transaction lifecycle

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RN-589