There's no Magento 2 Codex like there is for WordPress.
However, you can find the official documentation at the following link: http://devdocs.magento.com/
If you are learning Magento 2 I highly recommend the Magento U Fundamentals course that I took myself and helped me a lot understanding the software.
On top of that I recommend the Alan Storm articles that explain with deep investigations the concepts of Magento 2.
Try to get your hands on one of those books, it'll help you as well
Finally I suggest you use Magento StackExchange and Magento Forums as much as possible as you will get a lot of help from the community if you face any issue.