Is there actually any documentation covering all SPIs and injectable providers’ capabilities except the walkthrough here Server Developer Guide?
The extension documentation is lacking other than what you already mentioned. The Javadoc and others’ extensions is a good place to start. Is there a particular SPI you’re interested in?
Nothing specific, or better said - everything 
I’ll have to implement a lot of custom SPIs, but yeah, was just wondering whether there is any detailed documentation. Feels bad that the extension documentation isn’t that extensive, but I suppose whoever is implementing custom stuff has to look into the source code and Javadoc itself anyways.
Thanks a lot xgp
The best way to start is to use the documentation from Keycloak and of course check the keycloak code, most of the stuff you can find there. There is a lot of SPI’s so you won’t lack examples.