Add todo component developer documentation (#1929)

* Add todo component  developer documentation

* Update todo item property documentation

* Update table syntax

* Update developer documentation from core PR changes.

* Update the move item method signature

* Update docs/core/entity/todo.md

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>

* Improvements to todo developer documentation

* Apply suggestions from code review

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>

* Update TodoItem field requirements.

---------

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>

docs/core/entity/todo.md

@@ -0,0 +1,108 @@
+---
+title: To-do List Entity
+sidebar_label: To-do List
+---
+
+A To-do list entity is an entity that represents a To-do list. A To-do list contains
+To-do items which are ordered and have a status (complete or in progress). A To-do list entity is derived from the [`homeassistant.components.todo.TodoListEntity`](https://github.com/home-assistant/core/blob/dev/homeassistant/components/todo/__init__.py).
+
+## Properties
+
+:::tip
+Properties should only return information from memory and not do I/O (like network requests). Implement `update()` or `async_update()` to fetch data.
+:::
+
+| Name  | Type          | Default               | Description                                             |
+| ----- | ------------- | --------------------- | ------------------------------------------------------- |
+| todo_items | <code>list[TodoItem] &#124; None</code> | `None` | **Required.** The ordered contents of the To-do list. |
+
+### States
+
+A `TodoListEntity` state is the count of incomplete items in the To-do list.
+
+## Supported Features
+
+Supported features are defined by using values in the `TodoListEntityFeature` enum
+and are combined using the bitwise or (`|`) operator.
+
+| Value               | Description                                                        |
+| ------------------- | ------------------------------------------------------------------ |
+| `CREATE_TODO_ITEM`      | Entity implements the methods to allow creation of to-do items.  |
+| `DELETE_TODO_ITEM`      | Entity implements the methods to allow deletion of to-do items.  |
+| `UPDATE_TODO_ITEM`      | Entity implements the methods to allow update of to-do items.  |
+| `MOVE_TODO_ITEM`        | Entity implements the methods to re-order to-do items.  |
+
+## Methods
+
+
+### Create To-do items
+
+A To-do list entity may support creating To-do items by specifying the `CREATE_TODO_ITEM`
+supported feature.
+
+```python
+from homeassistant.components.todo import TodoListEntity
+
+class MyTodoListEntity(TodoListEntity):
+
+    async def async_create_todo_item(self, item: TodoItem) -> None:
+        """Add an item to the To-do list."""
+```
+
+### Delete To-do items
+
+A To-do list entity may support deleting To-do items by specifying the `DELETE_TODO_ITEM`
+supported feature. Integrations must support deleting multiple items.
+
+```python
+from homeassistant.components.todo import TodoListEntity
+
+class MyTodoListEntity(TodoListEntity):
+
+    async def async_delete_todo_items(self, uids: list[str]) -> None:
+        """Add an item to the To-do list."""
+```
+
+### Update To-do items
+
+A To-do list entity may support updating To-do items by specifying the `UPDATE_TODO_ITEM`
+supported feature. The `TodoItem` field `uid` is always present and indicates
+which item should be updated, and all other fields are optional. Integrations
+must support partial update.
+
+```python
+from homeassistant.components.todo import TodoListEntity
+
+class MyTodoListEntity(TodoListEntity):
+
+    async def async_update_todo_item(self, item: TodoItem) -> None:
+        """Add an item to the To-do list."""
+```
+
+### Move To-do items
+
+A To-do list entity may support re-ordering To-do items in the list by specifying
+the `MOVE_TODO_ITEM` supported feature. The To-do item with the specified `uid`
+should be moved to the position in the list specified by `pos` (`0` is the first
+position in the To-do list).
+
+```python
+from homeassistant.components.todo import TodoListEntity
+
+class MyTodoListEntity(TodoListEntity):
+
+    async def async_move_todo_item(self, uid: str, pos: int) -> None:
+        """Move an item in the To-do list."""
+```
+
+## TodoItem
+
+A `TodoItem` represents an individual item on a To-do list. The methods
+above describe any differences about which fields are optional on create or
+update.
+
+| Name        | Type             | Default      | Description                                                                                                                                     |
+| ----------- | ---------------- | ------------ | ----------------------------------------------------------------------------------------------------------------------------------------------- |
+| uid | <code>string &#124; None</code> | `None` | A unique identifier for the to-do item. This field is required for updates and the entity state.
+| summary     | <code>string &#124; None</code>  | `None` | A title or summary of the To-do item. This field is required for the entity state.
+| status | <code>TodoItemStatus &#124; None</code> | `None` | Defines the overall status for the To-do item, either `NEEDS_ACTION` or `COMPLETE`. This field is required for the entity state.