Add documentation for cowsay

Summary:
Cowsay is great, but what if you want to talk like a koala?

This diff adds diviner documentation on how to use the additional templates in externals/cowsay/cows/

Test Plan:

bin/diviner generate

https://my.phorge.site/book/phorge/article/differential_cowsay/

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25434