This feature is called entities and allows a group of characters to be replaced by only one character.
You can also add your own, change it or delete them. The actual conversions are listed in this file
And as pointed by @martinr you just needs to type the characters while editing and dokuwiki will do its job during rendering.
There is nothing to do with the HTML entities. I don't know why it's called entities, may be from the fact that the HTML entities are also a group of characters converted into one character.