Remove old doc page and add note about cache in installation
As discussed offline, removing the old page "Loading Google AI or OpenAI pre-trained weights or PyTorch dump" and move the not about cache in the installation folder (also, making that up to date ^^).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5027?src=pr&el=h1) Report
> Merging [#5027](https://codecov.io/gh/huggingface/transformers/pull/5027?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bbad4c6989d489097f42bbe38001a3f8ca1c5c11&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5027/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5027?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5027   +/-   ##
=======================================
  Coverage   77.19%   77.19%           
=======================================
  Files         128      128           
  Lines       21877    21877           
=======================================
+ Hits        16888    16889    +1     
+ Misses       4989     4988    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5027?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5027/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.57% <0.00%> (+0.15%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5027?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/5027?src=pr&el=footer). Last update [bbad4c6...2d3587c](https://codecov.io/gh/huggingface/transformers/pull/5027?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
