fix(explore): Update trace empty state docs link

[sentry-junior]

Updates the fallback “Go to Documentation” action in the Explore > Traces onboarding empty state to point at the Trace Explorer documentation instead of the retired Performance getting-started path.

Adds a regression test for projects without a tracing onboarding checklist.

Investigation notes:

• The variant is selected by useLoadGettingStarted: performance onboarding docs are loaded only for platforms in withPerformanceOnboarding.
• The full checklist intentionally composes the platform's performanceOnboarding install, configure, and verify steps; it is not based on JS versus non-JS directly.
• Existing analytics record onboarding exposure and docs clicks, including platform and checklist support.

Checks:

• git diff --check passed.
• Targeted Jest test not run: the dependency install was interrupted by the Slack turn cancellation.

Requested by Sasha Blumenfeld via Junior.

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