Refresh EDOT docs overview image and fix Collector sample link

[Copilot]

This updates two stale docs findings in the EDOT reference docs: a broken external link in the Collector compatibility page, and an outdated overview diagram on the reference landing page.

• Collector docs

  • Replaced the unresolved placeholder link to Collector sample configs with a live GitHub path in elastic/elastic-agent.
  • Updated the target to the current internal/pkg/otel/testdata directory, which now contains the relevant example configurations.

• Reference landing page

  • Refreshed docs/reference/images/EDOT-SDKs-Collector.png with the current EDOT overview diagram.
  • Replaced the generic image alt text with a descriptive summary of the telemetry flow shown in the diagram.

• Result

  • Removes the 404-producing external URL from the docs.
  • Brings the top-level EDOT architecture graphic in line with the current product layout.

- [example configuration files](https://github.com/elastic/elastic-agent/tree/main/internal/pkg/otel/testdata)

![Diagram showing infrastructure telemetry flowing through the EDOT Collector, application telemetry flowing through EDOT language SDKs, and both sending data to Elastic Observability](images/EDOT-SDKs-Collector.png)