Add documentation for rpc-client / features import

[haerdib]

This should include description on:

• Difference between rpc clients (e.g. when to use which client)
  • jsonrspee is only opens one websocket connection to the node (making it most likely the most efficient /fastest client). Offers the most convenience functions (subscription callbacks and such). Depends on tokio runtime.
  • ws client: Does not offer any convenience functions, but is simple and easily adaptable. Does not depend on tokio runtime.
  • tungstenite: offers (limited) reconnection possibilites. Other than that, similiar to ws-client.
• staking / contracts extrinsic import (see Make pallet-staking and pallet-contracts dependencies optional #522)