Reference complementary packages in the docs

[aleclarson]

For areas that Radashi could cover but someone already does it excellently, we should have a “Complementary Packages” section in the docs.

For color manipulation, recommend culori/fn.

[aleclarson]

Another one: date-fns

[aleclarson]

For common math needs: mz-math

(This one is new-ish, but appears to be well written. I'm open to more popular alternatives, if you have one.)