SpongeDocs is the official repository of documentation for the Sponge Poject.
The best way to become familiar with the Docs is to visit them, and read them in your language of choice:
https://docs.spongepowered.org/
If you want to help the SpongeDocs, there is a page of the Docs that explains how you can assist:
https://docs.spongepowered.org/stable/en/contributing/spongedocs.html
Contact SpongeDocs staff on GitHub / Discord / IRC / Sponge Forums if you have any further questions.