Expand a bit the presentation of examples
# What does this PR do?

This PR adds a bit more information to the examples README (main and specific per example), copying some information from the main philosophy and expanding a bit, to make sure all users know what we want for the examples.
I'd be super-handy to link directly to suitable datasets and models for each example as in 
- https://huggingface.co/datasets?search=squad
- https://huggingface.co/models?filter=squad

may be this could be an easy first good issue.

Some of the keywords and whether to use `?filter=` or `?search=` will require some investigation since the former is hidden and packs some power missing from the latter.<|||||>The first may be helpful, but the second is not necessarily: it shows the models that have been fine-tuned on a squad dataset, not the models that can be fine-tuned on it. There is no way to filter all the models that have an architecture containing a question-answering head as far as I know, which is what we would want to show.<|||||>Would this be at least in the right direction? https://huggingface.co/models?pipeline_tag=question-answering 
<|||||>Mmm, those seem to be models fine-tuned on a question-answering task, not all models with a QuestionAnswering arch available (for instance, you should see all BERT checkpoints, all distilBERT checkpoints etc).<|||||>OK, then it won't work.

It'd be really awesome if in the future we had a filter to filter models by architecture - and sub-architecture in this case - that is without the model-specific part of the class name.