Better examples
Refactored the examples section: removed old and not up-to-date examples and added new examples for fine-tuning and generation.

The `examples` file is not in the `/doc/source` folder anymore but in the `/examples` folder. It is therefore visible when users open the folder on GitHub. 

Note: In order to generate the current documentation, a symlink has to be done between the `examples/README.md` file to a `docs/source/examples.md`. The corresponding documentation has been added to the documentation README, alongside the command necessary to create the symlink.

**The new examples are visible on: http://lysand.re/examples.html**
There were indeed quite a few artifacts. I fixed them in the two latest commits.<|||||># [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1214?src=pr&el=h1) Report
> Merging [#1214](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1214?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/5ac8b62265efac24f0dbfab271d2bce534179993?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1214/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1214?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1214   +/-   ##
=======================================
  Coverage   81.29%   81.29%           
=======================================
  Files          57       57           
  Lines        8015     8015           
=======================================
  Hits         6516     6516           
  Misses       1499     1499
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1214?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/pytorch-transformers/pull/1214?src=pr&el=footer). Last update [5ac8b62...3f91338](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1214?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Awesome! Merging