I wanted to pick up Monogame, but I can't find documentation. All I see on GitHub is some tutorials. I'm looking for class reference like this from XNA.
MonoGame's documentation portal is here. They don't appear to currently have an API reference as of this writing.
I would venture to guess that this is because MonoGame's API tries to be identical to XNA's, so you can probably get away with just using XNA's API reference. The focus of the MonoGame project appears to be on longer-form documentation like tutorials and the like at the moment.
Monogame's documenation can be found on their site: http://monogame.net/documentation
They also have tutorials on their github page: https://github.com/mono/MonoGame/wiki/Tutorials
If you're looking for detailed documentation like Microsoft has, you're out of luck. Monogame is an opensource engine. Not a well funded company like Microsoft. Documentation is the last thing developers usually want to work on, so when they aren't being paid for it, it's the last thing (if ever) to get done.