There isn't really 1.7.10 documentation officially. The Forge documentation only documents the current version (the Forge contributors strongly want to encourage people to stay up to date, and won't even support questions on anything older than 1.10). But there are a bunch of tutorials out there, some of them are still relevant for 1.7.10. Also, the code has a fair amount of comments in it that should help you.
If you're still trying to do this your best bet is to find an older 1.7.10 opensource mod and try to understand how it works. I am also looking for a documentation and there is none but I figured it out quite well to the point that I can add mobs, worldgen, dimensions, blocks, items, models, renderers and more