docs(TreeSelect): simple docs to start (#1734)

src/components/TreeSelect/__stories__/Docs.mdx

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+import {Meta, Markdown} from '@storybook/addon-docs';
+
+import TreeSelectDocs from './TreeSelectDocs.md?raw';
+
+<Meta title="Lab/TreeSelect" />
+
+<Markdown>{TreeSelectDocs}</Markdown>

src/components/TreeSelect/__stories__/TreeSelectDocs.md

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+# TreeList
+
+Rewritten component [Select](https://preview.gravity-ui.com/uikit/?path=/docs/components-inputs-select--docs) without feature-specific logic using TreeList inside.
+
+`Storybook` provides complex examples how to use this components from this documentation.
+
+### Import:
+
+```tsx
+import {unstable_TreeSelect as TreeSelect} from '@gravity-ui/uikit/unstable';
+```
+
+### Basic example:
+
+```tsx
+import {
+  type unstable_ListItemType as ListItemType,
+  unstable_TreeSelect as TreeSelect,
+} from '@gravity-ui/uikit/unstable';
+
+const items: ListItemType<string>[] = ['one', 'two', 'free', 'four', 'five'];
+
+<TreeSelect items={items} mapItemDataToContentProps={(item) => ({title: item})} />;
+```
+
+### With full list item view override:
+
+```tsx
+import {Text} from '@gravity-ui/uikit';
+import {
+  type unstable_ListItemType as ListItemType,
+  type unstable_ListItemView as ListItemView,
+  unstable_TreeSelect as TreeSelect,
+} from '@gravity-ui/uikit/unstable';
+
+interface Entity {
+  id: string;
+  title: string;
+}
+
+const items: ListItemType<Entity>[] = [
+  {title: 'one', id: '1'},
+  {title: 'two', id: '2'},
+  {title: 'free', id: '3'},
+  {title: 'four', id: '4'},
+  {title: 'five', id: '5'},
+];
+
+const Component = () => {
+  return (
+    <TreeSelect
+      items={items}
+      getItemId={({id}) => id}
+      mapItemDataToContentProps={({title}) => ({title})}
+      renderItem={({data: {title}, props}) => (
+        <ListItemView {...props} content={<Text>{title}</Text>} />
+      )}
+    />
+  );
+};
+```