I cannot find any information how to embed a position anywhere in the help centre. Here would be a really obvious place for it to be: https://chess.stackexchange.com/editing-help
The only way I can figure out how to do it is to find an existing question with an embedded game, then view the source, and do the same thing.
Is it possible to get some basic documentation added to the help centre? Or even to the UI when you're asking a question?