docs(vue-testing-library): add slot usage examples (#1516)

Author: time1043

docs/vue-testing-library/api.mdx

@@ -78,6 +78,24 @@ Vue Router is passed, it will be used.
 
 It will be merged with `propsData`.
 
+##### `slots` (`Object`)
+
+Used to provide slot content when rendering the component. Keys are slot names, 
+and values can be strings, template snippets, or components, similar to how 
+[slots](https://test-utils.vuejs.org/api/#slots) are defined in Vue Test Utils.
+
+For example:
+
+```js
+render(MyComponent, {
+  slots: {
+    default: 'Default',
+    first: h('h1', {}, 'Named Slot'),
+    second: Bar
+  },
+})
+```
+
 ##### `container` (`HTMLElement`)
 
 By default, `Vue Testing Library` will create a `div` and append it to the