Add PHP to the AG-UI ecosystem: Neuron AI implements the protocol

Hi AG-UI team,

The documentation lists SDKs and integrations for several ecosystems, but PHP
is not represented anywhere. This gives the impression that PHP is outside the
AG-UI world, which is not accurate.

PHP has a working AG-UI implementation through Neuron AI, an open-source
framework for building AI agents in PHP. The integration was shown in a live
demo by Luca Lusso (SparkFabrik) during a recent webinar:

- Webinar recording: https://www.youtube.com/watch?v=5crd9XVlzjI
- Talk focus: "Backend-frontend integration protocols with Neuron AI: AG-UI,
agentic UIs, real-time Svelte widgets and a live demo."

Neuron AI:
- Website and docs: https://neuron-ai.dev
- Repository: https://github.com/neuron-core/neuron-ai

Would you consider adding PHP (via Neuron AI) to the ecosystem/integrations list
in the docs? It would help PHP developers discover that AG-UI support already
exists for them.

Thanks for the work on AG-UI.