add catalan data structures book (#8567)

* add catalan data structures book

* fix catalan markdown

* fix catalan language code and add more books

* fix catalan url

* fix section of proposicional logic book in catalan

* Rename free-programming-books-es_CA.md to free-programming-books-ca.md

* Update README.md

* flatten index

* space

Co-authored-by: Eric Hellman <eric@hellman.net>

README.md

@@ -87,6 +87,7 @@ This project lists books and other resources grouped by genres:
 + [Burmese / မြန်မာဘာသာ](books/free-programming-books-my.md)
 + [Chinese / 中文](books/free-programming-books-zh.md)
 + [Czech / čeština / český jazyk](books/free-programming-books-cs.md)
++ [Catalan / catalan/ català](books/free-programming-books-ca.md)
 + [Danish / dansk](books/free-programming-books-da.md)
 + [Dutch / Nederlands](books/free-programming-books-nl.md)
 + [Estonian / eesti keel](books/free-programming-books-et.md)