Fix broken links in the converting tf ckpt document
# What does this PR do?

This PR is to fix broken links in ["Converting TensorFlow Checkpoints"](https://huggingface.co/transformers/converting_tensorflow_models.html).

Advised by @LysandreJik in issue #9656, I updated the links.
I also refer to issue #8720 and it seems the issue is solved by this PR.

I think there are some outdated explanations.
As I discussed in issue #9657, I think it should be better to explain `from_pretrained()` instead of `torch.save()`.
Hence, I think the explanation below should be updated.

```
You can then disregard the TensorFlow
checkpoint (the three files starting with ``bert_model.ckpt``\ ) but be sure to keep the configuration file (\
``bert_config.json``\ ) and the vocabulary file (\ ``vocab.txt``\ ) as these are needed for the PyTorch model too.
```

I would be happy to get some advice on how to proceed with this PR.

Fixes #9656
Fixes #8720 


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

 albert, bert, XLM: @LysandreJik
I think the following comment of my own can be left as a PR for the future.
This change seems to require a modification of the `convert` code, and I think it may be necessary to separate it from this broken link issue.

I'm sorry for saying such a thing, even though the following is what I wrote.

> I think there are some outdated explanations.
> As I discussed in issue #9657, I think it should be better to explain `from_pretrained()` instead of `torch.save()`.
> Hence, I think the explanation below should be updated.
> 
> ```
> You can then disregard the TensorFlow
> checkpoint (the three files starting with ``bert_model.ckpt``\ ) but be sure to keep the configuration file (\
> ``bert_config.json``\ ) and the vocabulary file (\ ``vocab.txt``\ ) as these are needed for the PyTorch model too.
> ```
> 

I'll remove the WIP for now, but if you think this matter should be worked in this PR, please let me know.