[serve][docs] Add DAG building classes to the API reference

[edoakes]

• InputNode
• DAGDriver
• Various adapters

[cszhu]

This P2 issue has seen no activity in the past 2 years. It will be closed in 2 weeks as part of ongoing cleanup efforts.

Please comment and remove the pending-cleanup label if you believe this issue should remain open.

Thanks for contributing to Ray!