Make design guidelines around other ways to start a new chat

[rebeccaalpert]

The ShadowBot extension team was asking for other ways they could direct a user to start a new chat. This may also come up for InstructLab. Right now we only support this function as part of the chat history drawer. We suggested previously that the extension team add a dropdown item to the options menu or add a pencil icon button to the header. We should provide design guidelines to reflect this and make these suggestions available to other teams.