Where to find API document

Also I want to ask does NEI have a 1.12.2 Version for forge?

I have no idea about NEI or JEI, those projects are run by other people and I haven't paid attention to either of them so you probably know more than me!

As for CraftGuide:

Most of the interfaces defined in /src/main/java/uristqwerty/CraftGuide/api have javadoc comments.

There's also /wiki/API-usage

Some of the classes in /1.7.10/src/main/java/uristqwerty/CraftGuide/recipes might work okay as example usage, but many of them have accumulated clutter from dealing with multiple versions of the other mods, or still use older, less convenient API methods. Sadly, the most recent development has still been for 1.7.10, as back when I was last actively working on the mod I started to feel like I wanted to make API improvements, and I don't have any active modpacks on later Minecraft versions to test in or motivate me to keep working.

That does remind me, the best API usage examples would be a small compatibility mod I built up, to support whatever modpack I was playing. Mostly I didn't publish it (I'd want to at least look into the other mods' licenses, etc.), but if you want to look at the sources, I just uploaded them to https://uristqwerty.ca/files/cgext/cgext-src.7z