Rename :ref: role to :doc:

This is to be more compatible with other compilers like Sphinx, that
uses :ref: to create a CrossReference and :doc: to link to another
document.
Remove unused import
Always use the correct language (automatically)

It's not needed to use the DEFAULT_LANG config
Warning for twin slugs

If the :doc: role finds more than one post with the same slug, it will
use the first one, emmiting a WARNING.
CHANGES.txt updated with the :doc: role documentation