Update ONNX docs
# What does this PR do?

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This PR gives the ONNX part of the documentation a much needed update to:

* Explain why we have three ONNX configuration objects (`OnnxConfig`, `OnnxConfigWithPast`, and `OnnxSeq2SeqConfigWithPast`) and how to interpret them.
* Explain what we mean by the `--features` argument of the export CLI.
* Provide an end-to-end guide on how to export a custom model, using the DistilBERT implementation as an example. I chose this approach so readers can run actual code instead of relying on a hypothetical unsupported architecture.

I also removed the deprecated section that was based on the `convert_graph_to_onnx.py` script as it seems to confuse people on which API to use. I couldn't find clear instructions on how the deprecation cycle works for the documentation of scripts like `convert_graph_to_onnx.py`, so please let me know if I should re-instate this part of the docs.

I also tweaked the help messages in the `transformers.onnx` CLI to make things a bit clearer to the end user.

The diff on the docs is quite huge, so I recommend reading the file directly.
This is great, thanks for working on it @lewtun! Pinging @stevhliu and @sgugger for knowledge.<|||||>Thank you for the feedback @sgugger 🙏 !

I've now added the doctest `>>>` syntax to all the Python code blocks and removed the permalinks to the class references. OK to merge once the CI tests pass?