[Doc] Change img path to remove mxnet dependency

[Hzfengsy]

Move cat image url to tlcpack/web-data

cc @tqchen

[mshr-h]

@tvm-bot rerun

[mshr-h]

https://ci.tlcpack.ai/blue/organizations/jenkins/tvm-gpu/detail/PR-16582/3/tests

    def __init__(self, url, *args, **kwargs):
        LOGGER.info(f"Caught access to {url}")
        url = url.strip()
        if url not in URL_MAP and not url.startswith(BASE):
            # Dis-allow any accesses that aren't going through S3
            msg = (
                f"Uncaught URL found in CI: {url}. "
                "A committer must upload the relevant file to S3 via "
                "https://github.com/apache/tvm/actions/workflows/upload_ci_resource.yml "
                "and add it to the mapping in tests/scripts/request_hook/request_hook.py"
            )
>           raise RuntimeError(msg)
E           RuntimeError: Uncaught URL found in CI: https://github.com/tlc-pack/web-data/blob/main/testdata/cat.png. A committer must upload the relevant file to S3 via https://github.com/apache/tvm/actions/workflows/upload_ci_resource.yml and add it to the mapping in tests/scripts/request_hook/request_hook.py
tests/scripts/request_hook/request_hook.py:228: RuntimeError

It seems like a committer needs to upload the image file to S3. @Hzfengsy @tqchen @MasterJH5574 @yongwww