docs: add custom HTTP clients concept page and HTTPX guide (#658)

## Summary

- Basically documenting https://github.com/apify/apify-client-python/issues/416.

## Description

- Add concept page documenting the pluggable HTTP client architecture (default client config, abstract base classes, `HttpResponse` protocol, `call` method parameters)
- Add guide page with step-by-step HTTPX implementation example (sync + async)
- Add `ApiLink` component for linking to API reference from docs

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>