docs: update README banner to rebrand design system

Generated from the openrouter-web monorepo banner template
(scripts/repo-banners/ — see its AGENTS.md for the process).
Ink background, Volt accent, Gordita/Plus Jakarta Sans type, and the
canonical brand lockup, per the rebrand DESIGN.md.

Author: LukasParke

README.md

@@ -1,3 +1,5 @@
+![hero illustration](./assets/banner.png)
+
 # OpenRouter SDK
 
 The [OpenRouter SDK](https://openrouter.ai/docs/sdks/python) is a Python toolkit designed to help you build AI-powered features and solutions. Giving you easy access to 400+ models across providers in an easy and type-safe way.
@@ -311,4 +313,4 @@ To run the test suite, you'll need to set up your environment with an OpenRouter
 
    ```bash
    pytest
-   ```
+   ```