Add more issue templates and customize issue template chooser
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If you agree, I could also add a link to [Discussions](https://github.com/huggingface/datasets/discussions) in order to reinforce the use of Discussion to make Questions (instead of Issues).

I could also add some other templates: Bug, Feature Request,...
2021-03-25T09:46:05Z
https://github.com/huggingface/datasets/pull/2109#issuecomment-806509490
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2021-03-25T09:46:05Z
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@theo-m we wrote our same comments at the same time... 😉 
2021-03-25T09:48:31Z
https://github.com/huggingface/datasets/pull/2109#issuecomment-806510900
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When opening an issue, it is not evident for the users how to choose a blank issue template. There is a link at the bottom of all the other issue templates (`Don’t see your issue here? Open a blank issue.`), but this is not very visible for users. This is the reason why many users finally chose the `add-dataset` template instead (this is more visible) for issues that indeed are not requesting the addition of a new dataset.

~~With this PR, the default blank issue template would be as visible as the other templates (as the `add-dataset` template), thus making easier for the users to choose it.~~

With this PR:
- more issue templates, besides `add-dataset`, are added: `bug-report` and `feature-request`
- the issue template chooser is customized, so that it now includes a link to `Discussions` for questions