[hlenke]

Removes a broken example from the browser check examples page and adds a note about imports.

What was wrong

• The "Core Web Vitals Validation" example never finishes. I ran it with Playwright against a local test page. page.evaluate hangs until the 30 s test timeout: the PerformanceObserver is created after the page has loaded, so its callback never fires and the promise never resolves. It also waits on FID, which needs a real user click.
• None of the examples import test and expect, so copying one into a check gives test is not defined.

What changed

• I replaced the Web Vitals code with one sentence: Checkly collects Web Vitals automatically for every page a browser check visits. It links to the Performance & Web Vitals page.
• I added one line under the intro saying the examples leave out the @playwright/test import.

Not changed (noted on the ticket)
The rest of the page is generic sample code. The accessibility example's "contrast check" only compares two colors, and using @axe-core/playwright would be better. Rewriting the page is a bigger editorial job, so I left it for a follow-up.

vale and npm run check:links are clean.

Linear: DOC-150

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[hlenke]

Follow-up from review (F254): the accessibility example now uses @axe-core/playwright, which ships in runtimes 2025.04 (4.10.1) and 2026.04 (4.11.1), instead of a 'contrast check' that only compared two colors (240db66). Tested with Playwright 1.64.1 and axe 4.11.1: a clean page passed, a page with contrast, label, alt-text and lang issues failed, and example.com passed. It wasn't run as a Checkly browser check, because the browser runner isn't enabled on the local stack. F255 (rewrite the page with Checkly-specific examples) is editorial and is tracked in DOC-235.