Refactor issues with yaml
This PR:
- Refactors the issue templates using the newly introduced YAML issues.
- Introduces a `config.yml` for issues which enables a redirect to the forum
- Removes the benchmarking issue which has not been used at all and introduces noise in the issue selection process. With its removal, I'm aiming for a lower number of blank issues.

See below for the comparison across issues. To see the issue templates yourself and play with the `config.yml`, head over to [my fork](https://github.com/LysandreJik/transformers/issues/new/choose).

Old         |  New
:-------------------------:|:-------------------------:
Bug report (MD) | Bug report (YML)
<img width="659" alt="image" src="https://user-images.githubusercontent.com/30755778/163278600-b8fd6b21-88bd-426c-976c-5c93115c0e44.png">  |  <img width="660" alt="image" src="https://user-images.githubusercontent.com/30755778/163278655-8ad5ccd0-f7f9-41e4-8334-a5b59d10f55f.png">
Old feature request           |  New feature request
<img width="797" alt="image" src="https://user-images.githubusercontent.com/30755778/163278892-d7736e07-e66d-4a3c-a4d6-8b546f2efb43.png">  |  <img width="796" alt="image" src="https://user-images.githubusercontent.com/30755778/163278913-bd83c945-d125-4739-9eee-a7ff8eb5932b.png">

The new index will now contain additional links, including a link to the forum:

<img width="1044" alt="image" src="https://user-images.githubusercontent.com/30755778/163278991-c5ad6cd8-c08a-4a45-b3c7-560ad2c28180.png">

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks!

For the new bug report screenshot, looks like the section Who can help is not editable?

The new layout looks good! I would like to have a section 'Code Snippet' if it is not in the new layout yet. 🙂 (Haven't really tried it yet, just looking the screenshots)<|||||>> The new layout looks good! I would like to have a section 'Code Snippet' if it is not in the new layout yet. slightly_smiling_face (Haven't really tried it yet, just looking the screenshots)

@ydshieh, yes the reproducible code example is available in the issue, I just migrated the existing issues so everything is still included.

Thanks for your reviews, merging!