feat(onboarding): Add logs to react router onboarding

[AbhiPrasad]

resolves https://linear.app/getsentry/issue/LOGS-249/add-logs-to-react-router-onboarding
resolves https://linear.app/getsentry/issue/LOGS-290/add-verify-steps-to-react-router-js-onboarding

also adds tests and cleans up snippet code adding. Should match https://docs.sentry.io/platforms/javascript/guides/react-router/ better now.

[linear]

LOGS-249 Add logs to react router onboarding

LOGS-290 Add verify steps to react router (JS) onboarding

[seer-by-sentry]

Potential bug: The React Router onboarding doc uses an undefined `SentryLogAttribute`, which will cause a `ReferenceError` and crash the user's application during verification.

• Description: The code in the React Router getting started documentation at Sentry.logger.info uses SentryLogAttribute.string('test_loader_error'). The SentryLogAttribute object is not defined in the Sentry JavaScript SDK or imported anywhere in the codebase. When a user copies and executes this code snippet during the onboarding verification step, their application will throw a ReferenceError: SentryLogAttribute is not defined. This crash prevents users from successfully verifying their Sentry setup for React Router when logs are enabled, breaking a key part of the onboarding flow.
• Suggested fix: Remove the call to the undefined SentryLogAttribute.string() wrapper. The value for the action key should be a plain string, like 'test_loader_error', consistent with other JavaScript onboarding documentation and the Sentry SDK's API.
  _{severity: 0.8, confidence: 1.0}

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[chargome]

🙏