Update most URLs to docs.github.com (#1896)

Now that GitHub have moved their documentation from
developer.github.com, we should update our links. I have also tried to
update the call strings to their new format, but since it was done via
regex, some of them may not match exactly.

github/ContentFile.py

@@ -37,7 +37,7 @@ import github.Repository
 
 class ContentFile(github.GithubObject.CompletableGithubObject):
     """
-    This class represents ContentFiles. The reference can be found here https://developer.github.com/v3/repos/contents/#get-contents
+    This class represents ContentFiles. The reference can be found here https://docs.github.com/en/rest/reference/repos#contents
     """
 
     def __repr__(self):