This website contains the official documentation of Forge. There's not much right now, but it contains some basic information on getting started, blocks, blockstates and more.
There are a lot of tutorials available online, covering most of the basic modding stuff (blocks, items, tools). The MDK also has an example mod, containing the main mod file for a mod.
After the basic stuff, and you a may be going to try more advanced stuff (furnaces, chests), you should've picked up the ability the search the Minecraft and Forge code base to figure out stuff without tutorials. If you're running into issues, make a post on Modder Support.