Docs - Catalan translation

Docs need translation in different languages. This issue covers the translation in Catalan.
How to do it: http://pygame-zero.readthedocs.io/en/latest/contributing.html#translating
Repo with the translations: https://github.com/BibianaC/pgzero-catalan

Start with the translating section of the contributing guide linked in the issue, then inspect the pgzero-catalan repository to understand its current translation structure. Complete the Catalan documentation translation there and verify that the resulting translated content is available through the documented process.

Written by the indexing model from the issue text.