doc: add diagram for how PHP SDK interacts with Dapr runtime

## Describe the proposal

**Is your feature request related to a problem? Please describe.**
There is no visual representation for how the SDK interacts with the Dapr runtime.

**Describe the solution you'd like**
It would be nice for community members to have a diagram depicting the interactions with end user applications, the Dapr PHP SDK, and the Dapr sidecar close to the codebase, within the README.md file.

The diagram should include:

- 3 main components: end user application, Dapr PHP SDK, Dapr sidecar:
- Initialization: which protocol is the application Dapr client using to establish connection with the Dapr sidecar, HTTP or gRPC? That depends on the SDK and should be depicted visually.
- API calls that one can use the Dapr client for. For example you can save & retrieve state from state stores, publish and subscribe to events using pub/sub, invoke other services using service invocation, etc. We should represent which APIs the SDK supports.
- Overall request routing (when app makes API call through Dapr client, then the request is sent to the Dapr sidecar running along the app)

Other ideas that can be included in diagram:

- Sidecar processing
- Response handling
- Middleware and observability
- Error handling
- Security

**Describe alternatives you've considered**
We could technically keep the README.md file as is, but it would be nice to have visuals here for community members understanding of these interactions.

**Additional context**
https://docs.dapr.io/developing-applications/sdks/