Is there an option to align text (and also an image) to the center in sanity.io's rich text editor (Portable text)? Couldn't find anything in the docs
No, there is not yet an out-of-the-box solution. Sanity has many requests for this. But you could make one yourself based on css. More info on it here. You might get quicker support on the Sanity Slack channels though. There are also a few existing community align approaches if you use search there.