Add documentation for LoadArtifactsTool

[hangfei]

Right now, i didn't find documentation for https://github.com/google/adk-python/blob/16392984c51b02999200bd4f1d6781d5ec9054de/src/google/adk/tools/load_artifacts_tool.py#L31. This is useful and we should add documentation for it.

[wuliang229]

+1. I was adding a test for the load_artifacts tool and I didn't find any documentation except for the example agent in https://github.com/google/adk-python/tree/main/contributing/samples/generate_image. I think https://google.github.io/adk-docs/artifacts/ would be a good candidate page to mention the tool.