docs: document Playwright browser plugin extensions (#2089)

### 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 https://github.com/apify/crawlee-python/issues/1936