docs(tanstackstart): Add docs for distributed tracing

[JPeer264]

DESCRIBE YOUR PR

Follow up to getsentry/sentry-javascript#20649 (comment)

IS YOUR CHANGE URGENT?

Help us prioritize incoming PRs by letting us know when the change needs to go live.

• Urgent deadline (GA date, etc.):
• Other deadline:
• None: Not urgent, can wait up to 1 week+

SLA

• Teamwork makes the dream work, so please add a reviewer to your PRs.
• Please give the docs team up to 1 week to review your PR unless you've added an urgent due date to it.
  Thanks in advance for your help!

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)
• PR was reviewed and approved by a member of the Sentry docs team

LEGAL BOILERPLATE

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

EXTRA RESOURCES

• Sentry Docs contributor guide

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
sentry-docs	Ready	Preview, Comment	May 12, 2026 0:11am

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
develop-docs	Ignored		May 12, 2026 0:11am

[sentry]

Bug: The Sentry.init code snippet for TanStack Start uses an undefined router variable, which will cause a ReferenceError if copied directly.
_{Severity: HIGH}

Suggested Fix

Update the documentation to show the Sentry.init call within the correct file context, such as src/router.tsx inside the getRouter() function, where the router constant is defined. Alternatively, add a note explaining where the router variable comes from and that the snippet needs to be placed in the appropriate file where router is in scope.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location:
platform-includes/distributed-tracing/how-to-use/javascript.tanstackstart-react.mdx#L65-L72

Potential issue: The documentation snippet in
`platform-includes/distributed-tracing/how-to-use/javascript.tanstackstart-react.mdx`
demonstrates initializing Sentry using
`Sentry.tanstackRouterBrowserTracingIntegration(router)`. However, the `router` variable
is not defined or imported in the provided example file context
(`sentry.client.config.ts`). When a developer follows this guide, the application will
throw a `ReferenceError: router is not defined` at runtime because the `router` instance
is not available in that scope. The correct initialization should occur where the router
is created, typically within `src/router.tsx`.

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