Mention NumCL on arrays.md

[vindarel]

from https://github.com/LispCookbook/cl-cookbook/pull/401

for @bendudson or @alx-a (or me) ;)

It might good to add a link to NumCL to this page (https://github.com/numcl/numcl), and think about which library this page should point new users to. At the moment it's mainly using examples from array-operations because that seemed relatively straightforward and covered a lot of common use-cases. Has NumCL now reached a state where it can be recommended as the place new users should start?