docs: document Playwright browser plugin extensions

[shixi-li]

Description

The Playwright crawler guide linked a custom implementation without explaining the extension contract, making it unclear when configuration is sufficient and when a different launch API needs a plugin subclass.

• distinguish built-in configuration from custom browser launch integrations
• document the new_browser() controller contract and BrowserPool lifecycle responsibilities
• call out context, incognito, page-limit, fingerprint, header, and user-data behavior that custom integrations must preserve or replace
• link the existing Camoufox implementation as a complete example

Issues

Refs #1936

Testing

• uv run --locked poe lint
• cd website && corepack pnpm@11.15.1 format:check
• cd website && corepack pnpm@11.15.1 lint:code
• cd website && corepack pnpm@11.15.1 lint:md
• cd website && uv run --locked corepack pnpm@11.15.1 build

Checklist

• CI passed