Improve documentation with information about FileArtifactService

[Spikhalskiy]

The documentation pages do not mention FileArtifactService support for local file storage.
https://google.github.io/adk-docs/api-reference/cli/cli.html mentions

--artifact_service_uri <artifact_service_uri>
Optional. The URI of the artifact service, supported URIs: gs://<bucket name> for GCS artifact service.

This section should contain information about how to use FileArtifactService. (file://<root_path>)