Go quickstart documents API key auth only, with no Application Default Credentials path

The Go quickstart at https://adk.dev/get-started/go/ covers a single authentication method. Its auth section is titled "Set your API key", the sample uses `APIKey: os.Getenv("GOOGLE_API_KEY")`, and readers are sent to AI Studio for a key. Application Default Credentials are not mentioned, and there is no link from that section to the Google Cloud page.

ADK Go supports Application Default Credentials as a first-class option. `auth.ADC()` is an exported credential provider alongside `auth.APIKey()` and `auth.ServiceAccount()` in `auth/providers.go`, defaulting to the `cloud-platform` scope. Passing `Backend: genai.BackendVertexAI` to `gemini.NewModel` authenticates through Application Default Credentials with no key at all, which several examples in the repository already do. Application Default Credentials are also used for telemetry export, the agent registry, and the GCP auth client.

The "Connect to Google Cloud and Agent Platform" page at https://adk.dev/get-started/google-cloud/ covers this well, leading with Application Default Credentials and placing the API key path last. A reader who follows the Go quickstart start to finish never reaches it.

This matters most for organizations that will not distribute API keys, who currently have to discover the supported path by reading the source. Adding a short alternative-auth subsection to the Go quickstart, or a pointer to the Google Cloud page from the auth section, would close the gap.

Start with the Go quickstart auth section at https://adk.dev/get-started/go/ and compare it with the Google Cloud guide at https://adk.dev/get-started/google-cloud/. Review auth/providers.go and the existing Go examples using genai.BackendVertexAI, then update the quickstart so readers can find the Application Default Credentials path and its Google Cloud guidance.

Written by the indexing model from the issue text.