Docs: migration guide — from Selenium + TestNG #9

Migration searches are high-intent — someone searching "migrate selenium testng framework" is close to adopting. This is likely to become a top-traffic page.

Task

Write docs-site/docs/migration/from-selenium-testng.md: a side-by-side "your current setup -> Selenium Boot equivalent" that shows how much boilerplate disappears (driver factory, waits utils, retry analyzer, reporting).

Acceptance criteria

• Before/after code for the common cases (driver setup, waits, retry, reporting)
• description: frontmatter; linked in sidebar
• npm run build passes