Add the main framework for the OptionList documentation

Lots to flesh out here still, but this gets the core framework in place.

docs/widgets/selection_list.md

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+# SelectionList
+
+!!! tip "Added in version 0.??.0"
+
+A widget for showing a vertical list check boxes.
+
+- [x] Focusable
+- [ ] Container
+
+## Examples
+
+Some super-cool examples will appear here!
+
+## Reactive Attributes
+
+| Name          | Type            | Default | Description                                                                  |
+|---------------|-----------------|---------|------------------------------------------------------------------------------|
+| `highlighted` | `int` \| `None` | `None`  | The index of the highlighted selection. `None` means nothing is highlighted. |
+
+## Messages
+
+The following messages will be posted as the user interacts with the list:
+
+- [SelectionList.SelectionHighlighted][textual.widgets.SelectionList.SelectionHighlighted]
+- [SelectionList.SelectionToggled][textual.widgets.SelectionList.SelectionToggled]
+
+The following message will be posted if the content of
+[`selected`][textual.widgets.SelectionList.selected] changes, either by user
+interaction or by API calls:
+
+- [SelectionList.SelectedChanged][textual.widgets.SelectionList.SelectedChanged]
+
+## Bindings
+
+The selection list widget defines the following bindings:
+
+::: textual.widgets.SelectionList.BINDINGS
+    options:
+      show_root_heading: false
+      show_root_toc_entry: false
+
+It inherits from [`OptionList`][textual.widgets.OptionList]
+and so also inherits the following bindings:
+
+::: textual.widgets.OptionList.BINDINGS
+    options:
+      show_root_heading: false
+      show_root_toc_entry: false
+
+## Component Classes
+
+The selection list provides the following component classes:
+
+::: textual.widgets.SelectionList.COMPONENT_CLASSES
+    options:
+      show_root_heading: false
+      show_root_toc_entry: false
+
+It inherits from [`OptionList`][textual.widgets.OptionList] and so also
+makes use of the following component classes:
+
+::: textual.widgets.OptionList.COMPONENT_CLASSES
+    options:
+      show_root_heading: false
+      show_root_toc_entry: false
+
+::: textual.widgets.SelectionList
+    options:
+      heading_level: 2
+
+::: textual.widgets.selection_list.Selection
+    options:
+      heading_level: 2
+
+::: textual.widgets.selection_list
+    options:
+      heading_level: 2