task: @Astrox_Network's "agent_dart" api doc
To have better understanding of internet computer, for client developers, it is the best way to understand how the sdk works.
Though we have ported most agent-js features to agent_dart, we don't have enough resources to complete the api docs. Which is really important work for community developers.
Thus, we need to cover as many apis as possible, we need someone to comment on code like we do for dart code.