* 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>
