Add version control menu
This PR adds at the top of the navigation bar a menu to pick a version of the docs.

A few comments:

When switching version, the reader is sent on the same page of the docs in the older version (so it gives an error if the same page did not exist in this version of the docs). I don't know if this is preferable to the alternative (sending back to the index of the other version in the docs). Let me know what you think.

The menu will disappear once the reader goes to an older version of the docs (since it did not exist back then) unless we find a way to cherry pick in each release (but I doubt it's worth it).

Preview is [here](https://51918-155220641-gh.circle-artifacts.com/0/docs/_build/html/index.html). Side note, on a local build (like this one), the version appears as 'html', but it will be the right one once merged. Also, the preview/local build only has one version, so the links don't work there.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5222?src=pr&el=h1) Report
> Merging [#5222](https://codecov.io/gh/huggingface/transformers/pull/5222?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c439752482759c94784e11a87dcbf08ce69dccf3&el=desc) will **decrease** coverage by `0.10%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5222/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5222?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5222      +/-   ##
==========================================
- Coverage   78.07%   77.97%   -0.11%     
==========================================
  Files         138      138              
  Lines       23786    23786              
==========================================
- Hits        18572    18547      -25     
- Misses       5214     5239      +25     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5222?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5222/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-5.42%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5222/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `79.51% <0.00%> (-1.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5222/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `88.05% <0.00%> (-1.26%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5222/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `78.61% <0.00%> (-0.20%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5222/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.86% <0.00%> (-0.15%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5222?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/5222?src=pr&el=footer). Last update [c439752...12b85f4](https://codecov.io/gh/huggingface/transformers/pull/5222?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>nice. In terms of UI you could also just have used a `<select>` element (maybe slightly more explicit UI) but I guess this works too