Fix documentation links always pointing to master. (#9217)

* Use extlinks to point hyperlink with the version of code

* Point to version on release and master until then

* Apply style

* Correct links

* Add missing backtick

* Simple missing backtick after all.

Co-authored-by: Raghavendra Sugeeth P S <raghav-5305@raghav-5305.csez.zohocorpin.com>
Co-authored-by: Lysandre <lysandre.debut@reseau.eseo.fr>

docs/source/conf.py

@@ -27,7 +27,10 @@ author = u'huggingface'
 version = u''
 # The full version, including alpha/beta/rc tags
 release = u'4.1.1'
-
+# Prefix link to point to master, comment this during version release and uncomment below line
+extlinks = {'prefix_link': ('https://github.com/huggingface/transformers/blob/master/%s', '')}
+# Prefix link to always point to corresponding version, uncomment this during version release
+# extlinks = {'prefix_link': ('https://github.com/huggingface/transformers/blob/v'+ release + '/%s', '')}
 
 # -- General configuration ---------------------------------------------------
 
@@ -40,6 +43,7 @@ release = u'4.1.1'
 # ones.
 extensions = [
     'sphinx.ext.autodoc',
+    'sphinx.ext.extlinks',
     'sphinx.ext.coverage',
     'sphinx.ext.napoleon',
     'recommonmark',