books: add cookbook cover file

LispLang / lisplang.github.io

Common Lisp homepage

https://lisp-lang.org/

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Closed vindarel closed 3 years ago

vindarel commented 3 years ago

Hello,

this should fix the cookbook cover not displaying with the URL (sorry again) (broken ruby, can't test :S )

Best,

eudoxia0 commented 3 years ago

Thanks!