Quick tour
This PR replaces the old quickstart guide by splitting it in two:
- a quick tour that is before the installation page,
- the philosophy page, after the installation.

It also renames Usage to Task summary since it's really what it is, and renames the files usage to task_summary, summary to model_summary.

The quick tour tries to go over all the API introduced in Transformers at a high level, pointing out tutorials (existing or upcoming) for each of them.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5145?src=pr&el=h1) Report
> Merging [#5145](https://codecov.io/gh/huggingface/transformers/pull/5145?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f45e873910e60d89511ae0193711e71c5c710468&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5145/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5145?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5145   +/-   ##
=======================================
  Coverage   77.19%   77.19%           
=======================================
  Files         133      133           
  Lines       22233    22233           
=======================================
  Hits        17163    17163           
  Misses       5070     5070           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5145?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5145/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5145/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.96% <0.00%> (+0.14%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5145?src=pr&el=footer). Last update [f45e873...ae14c1a](https://codecov.io/gh/huggingface/transformers/pull/5145?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>That's awesome, I think this PR is really cool! I wonder whether A short section showing a super simple training example should already be in the quicktour<|||||>For the quick training example, I have it in mind but for when the integration with nlp is smooth: gathering the data takes a bit too long in code right now, and we have to talk about some obscure function that prepares it, which doesn't really fit the rest. 