Is there something like https://developer.wordpress.org/reference/ (Code Reference) for Magento CE v.1.x like this is for WordPress?
There's the API references in the official documentation: http://devdocs.magento.com/guides/m1x/
Regarding code reference, I'm not aware of any official resource unfortunately.
There is actually a Magento 1.x Reference, but it's not quite the same as the one from WordPress.
The Magento 1.x reference is mostly about installations, upgrades, patches, and customizations: http://devdocs.magento.com/guides/m1x/.
I remember there was also once a documentation of classes and methods online. The most similar thing I could find right now is this: http://devdocs.magento.com/guides/m1x/.
Anyway, as a Magento developer I'd say nothing's a better documentation than your local source code :-)