Fix documentation links always pointing to master.
# What does this PR do?

This PR fixes documentation issue. The issue is that hyperlinks pointing to code file in the docs always point to the master.
Reference issue : https://github.com/huggingface/transformers/issues/7988

I found [extlinks](https://www.sphinx-doc.org/en/master/usage/extensions/extlinks.html) more appropriate than [rst_epilog
](https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-rst_epilog) as mentioned in the original issue because hyperlink replacement is not possible with `rst_epilog` 
Stackoverflow issue reference : https://stackoverflow.com/questions/1227037/substitutions-inside-links-in-rest-sphinx

Although this fixes the issues as discussed I am unable to fix the  issue  in markup files which are https://huggingface.co/transformers/master/contributing.html and https://huggingface.co/transformers/master/notebooks.html
Please suggest if it is possible to do that as well. 


Fixes https://github.com/huggingface/transformers/issues/7988

@sgugger @LysandreJik 

Thanks for merging @LysandreJik but I want to know why the errors occurred when I ran `make style ` as I see in https://github.com/huggingface/transformers/pull/9217#discussion_r548898170