docs(javascript): Update sentry-testkit capabilities

[zivl]

DESCRIBE YOUR PR

Updates the Sentry Testkit best-practices page to reflect the library's current capabilities. The page previously only documented error reports and transactions; sentry-testkit now captures a much wider range of Sentry event types and supports newer SDK versions.

• Note support for the Sentry JavaScript SDK v8, v9, and v10
• Add a "What You Can Capture" section covering structured logs, user feedback, and cron check-ins (with a note that the list keeps growing)
• Document finding/filtering helpers (findReport, findReportByMessage, findTransaction, reportsWithTag, transactionsWithTag, isExist)
• Document the awaitable waitFor* helpers for asynchronous events
• Add a "Network Interception" section for testing without overriding the transport (E2E, Puppeteer, Playwright, browser-only, React Native)
• Add the testkit.reset() between-tests pattern
• Point usage-example links to the maintained docs site (https://zivl.github.io/sentry-testkit) instead of the GitHub repo

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• 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

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