Handbook/mirrors update

Hi.

The mirrors section in the Handbook has been updated [1]. We had before
much stale information.

Several mirrors were removed [2] from the list, either is not responding
or redirecting (DNS) to the official mirrors, and links for each
supported protocol were added to help users. In case of any error,
please let us know.

And now there is more information about the official mirrors of the
project.

Regards.

1 - https://docs.freebsd.org/en/books/handbook/mirrors/
2 - https://people.freebsd.org/~dbaio/mirrors/mirrors-lists-removed.txt

-- 
Danilo G. Baio (dbaio)