[ASR Examples] Update README for Whisper
# What does this PR do?

Summarises changes from https://github.com/huggingface/transformers/pull/19519 and the blog post at https://huggingface.co/blog/fine-tune-whisper, providing examples for fine-tuning Whisper using the example script `run_speech_recognition_seq2seq.py`.

Required some re-jigging of the Speech-Encoder-Decoder Model examples in the section "[Automatic Speech Recognition with Sequence-to-Sequence](https://github.com/huggingface/transformers/pull/20230#sequence-to-sequence)".
_The documentation is not available anymore as the PR was closed or merged._