Updates to the templates
This PR updates the existing GitHub templates. Main changes are:

- motivate users to post general question on Stack Overflow, tagged [huggingface-transformers](https://stackoverflow.com/questions/tagged/huggingface-transformers)
- removing the 'additional context' section as it might not add much and just bloats the template
- changed references to pytorch-transformers to transformers

closes https://github.com/huggingface/transformers/issues/2529
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2664?src=pr&el=h1) Report
> Merging [#2664](https://codecov.io/gh/huggingface/transformers/pull/2664?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ea2600bd5f1d36f2fb61958be21db5b901e33884?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2664/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2664?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2664   +/-   ##
=======================================
  Coverage   74.51%   74.51%           
=======================================
  Files          87       87           
  Lines       14920    14920           
=======================================
  Hits        11117    11117           
  Misses       3803     3803
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2664?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2664?src=pr&el=footer). Last update [ea2600b...1cfc4af](https://codecov.io/gh/huggingface/transformers/pull/2664?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
