Add list view and docs

docs/widgets/list_view.md

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+# List View
+
+Displays a vertical list of widgets which can be highlighted and selected.
+Supports keyboard navigation.
+
+- [x] Focusable
+- [x] Container
+
+## Example
+
+The example below shows an app with a simple `ListView`.
+
+=== "Output"
+
+    ```{.textual path="docs/examples/widgets/list_view.py"}
+    ```
+
+=== "list_view.py"
+
+    ```python
+    --8<-- "docs/examples/widgets/list_view.py"
+    ```
+
+## Reactive Attributes
+
+| Name    | Type  | Default | Description                     |
+|---------|-------|---------|---------------------------------|
+| `index` | `int` | `0`     | The currently highlighted index |
+
+## Messages
+
+### Highlighted
+
+The `ListView.Highlighted` message is emitted when the highlight changes.
+This happens when you use the arrow keys on your keyboard and when you
+click on a list item.
+
+- [x] Bubbles
+
+#### Attributes
+
+| attribute | type       | purpose                        |
+|-----------|------------|--------------------------------|
+| `item`    | `ListItem` | The item that was highlighted. |
+
+### Selected
+
+The `ListView.Selected` message is emitted when a list item is selected.
+You can select a list item by pressing ++enter++ while it is highlighted,
+or by clicking on it.
+
+- [x] Bubbles
+
+#### Attributes
+
+| attribute | type       | purpose                     |
+|-----------|------------|-----------------------------|
+| `item`    | `ListItem` | The item that was selected. |
+
+## See Also
+
+* [ListView](../api/list_view.md) code reference