Correct paths in documentation URLs (#4259)

* Correct paths in documentation URLs

These changes update documentation links to their correct URLs.

* Fix anchor

Should have validated the full URL earlier!

README.rst

     :target: http://oscarcommerce.com
 
 .. image:: https://github.com/django-oscar/django-oscar/raw/master/docs/images/screenshots/readthedocs.thumb.png
-    :target: https://django-oscar.readthedocs.io/en/stable/
+    :target: https://django-oscar.readthedocs.io/en/latest/
 
 Further reading:
 
 The sandbox site can be set-up locally `in 5 commands`_.  Want to
 make changes?  Check out the `contributing guidelines`_.
 
-.. _`in 5 commands`: https://django-oscar.readthedocs.io/en/stable/internals/sandbox.html#running-the-sandbox-locally
-.. _`contributing guidelines`: https://django-oscar.readthedocs.io/en/stable/internals/contributing/index.html
+.. _`in 5 commands`: https://django-oscar.readthedocs.io/en/latest/internals/sandbox.html#run-the-sandbox-locally
+.. _`contributing guidelines`: https://django-oscar.readthedocs.io/en/latest/internals/contributing/index.html
 
 
 Extensions