[Longformer] Docs and clean API
This PR:

- adds a documentation page for Longformer. @ibeltagy - it's best to read it using this link I think: https://github.com/huggingface/transformers/pull/4464/files?short_path=3909947#diff-3909947f36862a1731195bf05c85c64c. 

- fixes a typo to correctly render the pretrained models doc page

- changes the API of Longformer slightly. I removed the `attention_mode` from Longformer because I don't think it should be used. The modus should always be `Longformer` since it is a `Longformer` model. The user should not be able to create a `RobertaModel` using `LongformerModel`. 
For comparisons people should use `RobertaModel` vs `LongformerModel` and not different modi of Longformer which is essentially the same as `RobertaModel` (correct me if I'm wrong here @ibeltagy).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4464?src=pr&el=h1) Report
> Merging [#4464](https://codecov.io/gh/huggingface/transformers/pull/4464?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8f1d0471489352ec01556ae61f8e8246002bbc58&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4464/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4464?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4464      +/-   ##
==========================================
+ Coverage   77.93%   77.98%   +0.04%     
==========================================
  Files         123      123              
  Lines       20430    20426       -4     
==========================================
+ Hits        15922    15929       +7     
+ Misses       4508     4497      -11     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4464?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4464/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4464/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `82.94% <100.00%> (+0.18%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4464/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.41% <0.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4464/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (+1.80%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4464?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4464?src=pr&el=footer). Last update [8f1d047...e53e5dc](https://codecov.io/gh/huggingface/transformers/pull/4464?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
