Change transformers.onnx to use optimum.exporters.onnx
# What does this PR do?

As the title say. The `transformers.onnx` command-line tool now uses the `optimum.exporters.onnx` command-line tool in the background, and redirects the user to use this tool directly for the next times (same in the documentation).
_The documentation is not available anymore as the PR was closed or merged._