Link isort profile to Black code style isort mention (#2246)

The isort configuration currently in the Black code style document is
duplicated in Using Black with other tools document. I think it would
be better to consolidate information and simply link to the tool guide,
mentioning the easy profile in the original document.

I changed the link from isort PyPI page to Black's docs on isort
because for users it could be better to see the Black docs on why that
configuration is necessary and what isort is from Black's perspective.