add Content
Adds a single Content to Builder.content.
Throws
if the field was previously set to a non-list.
Alias for calling addContent with Content.ofText(text).
Alias for calling addContent with Content.ofImage(image).
Alias for calling addContent with Content.ofAudio(audio).
Alias for calling addContent with Content.ofDocument(document).
Alias for calling addContent with Content.ofVideo(video).