[docs] Add widgets customization page

.changeset/nice-rings-divide.md

@@ -0,0 +1,5 @@
+---
+"docs": patch
+---
+
+Add widgets customization page

apps/docs/src/content/docs/customization/widgets.mdx

@@ -0,0 +1,44 @@
+---
+title: Custom widgets
+sidebar:
+  order: 1
+---
+
+## Example
+
+```typescript
+import type { HTMLInputAttributes } from 'svelte/elements';
+
+import type { Widget, WidgetCommonProps, Widgets, WidgetType } from '@sjsf/form';
+import { theme } from '@sjsf/form/basic-theme';
+
+import ToggleWidget from './toggle-widget.svelte';
+
+declare module '@sjsf/form' {
+	export interface WidgetsAndProps<V> {
+		toggle: WidgetCommonProps<V, HTMLInputAttributes>;
+	}
+
+	export interface WidgetValue {
+		toggle: boolean;
+	}
+}
+
+// Creating new theme
+const registry: { [T in WidgetType]: Widget<T> } = {
+	...,
+	toggle: ToggleWidget
+};
+// @ts-expect-error
+export const widgets: Widgets = (type) => registry[type];
+
+
+// Extension of existing theme
+// @ts-expect-error
+export const widgets: Widgets = (type, config) => {
+  if (type === "toggle") {
+    return ToggleWidget
+  }
+  return theme.widgets(type, config)
+}
+```