Honestly, I don't think Hubitat has good developer documentation at the moment, just the above that describes at least the signatures of most Hubitat-specific methods. The platform runtime environment is nearly identical to that of SmartThings, so if you're familiar with that you already know most. If you don't, their docs are really mostly a better resource, but use their "classic" docs (where things ran/run in the ST cloud; they are slowly moving to a model where you need to provide your own with an accessible endpoint). For SmartApps, see: SmartApp — SmartThings Classic Developer Documentation. For DTHs (like Hubitat drivers), see: Device Handlers — SmartThings Classic Developer Documentation.
There are some differences, mostly in driver land, which you can see most of here: App and driver porting to Hubitat
