#2913: How-To guide initial commit.

docs/source/how_to_guides/extensible_rewards.rst

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+.. only:: comment
+
+    © Crown-owned copyright 2024, Defence Science and Technology Laboratory UK
+
+.. _about:
+
+Extensible Rewards
+******************
+
+Changes to reward class structure.
+==================================
+
+Reward classes are inherited from AbstractReward (a sub-class of Pydantic's BaseModel).
+Within the reward class is a ConfigSchema class responsible for ensuring config file data is in the
+correct format. The `.from_config()` method is generally unchanged.
+
+Inheriting from `BaseModel` removes the need for an `__init__` method bu means that object
+attributes need to be passed by keyword.
+
+.. code:: Python
+
+class AbstractReward(BaseModel):
+    """Base class for reward function components."""
+
+    class ConfigSchema(BaseModel, ABC):
+        """Config schema for AbstractReward."""
+
+        type: str
+
+    _registry: ClassVar[Dict[str, Type["AbstractReward"]]] = {}
+
+    def __init_subclass__(cls, identifier: str, **kwargs: Any) -> None:
+        super().__init_subclass__(**kwargs)
+        if identifier in cls._registry:
+            raise ValueError(f"Duplicate node adder {identifier}")
+        cls._registry[identifier] = cls
+
+    @classmethod
+    def from_config(cls, config: Dict) -> "AbstractReward":
+        """Create a reward function component from a config dictionary.
+
+        :param config: dict of options for the reward component's constructor
+        :type config: dict
+        :return: The reward component.
+        :rtype: AbstractReward
+        """
+        if config["type"] not in cls._registry:
+            raise ValueError(f"Invalid reward type {config['type']}")
+        adder_class = cls._registry[config["type"]]
+        adder_class.add_nodes_to_net(config=adder_class.ConfigSchema(**config))
+        return cls
+
+    @abstractmethod
+    def calculate(self, state: Dict, last_action_response: "AgentHistoryItem") -> float:
+        """Calculate the reward for the current state.
+
+        :param state: Current simulation state
+        :type state: Dict
+        :param last_action_response: Current agent history state
+        :type last_action_response: AgentHistoryItem state
+        :return: Reward value
+        :rtype: float
+        """
+        return 0.0
+
+
+Changes to YAML file.
+=====================
+.. code:: YAML
+
+    There's no longer a need to provide a `dns_server` as an option in the simulation section
+    of the config file.