doc(queryconfig): Allows configuring a different element query mechanism

Provides the ability to specify a different element query mechanism other than querySelector and querySelectorAll.

An example of the usage with Georgegriff/query-selector-shadow-dom looks like this:

import { querySelectorAllDeep, querySelectorDeep } from 'query-selector-shadow-dom';
configure({
    queryElement: querySelectorDeep,
    queryAllElements: querySelectorAllDeep,
})
After this line, a query will also drill into the shadow dom of elements on the page

Why:

In our project, we are using a design system build with web components
(via stenciljs), and different projects in different frameworks (lit,
react, vue) want to use this library. The corresponding pr can be found
here:
testing-library/dom-testing-library#1054

Author: MatthiasKainer

docs/dom-testing-library/api-configuration.mdx

@@ -116,3 +116,39 @@ message and container object as arguments.
 
 The global timeout value in milliseconds used by `waitFor` utilities. Defaults
 to 1000ms.
+
+### `queryElement` & `queryAllElements`
+
+If your application requires a specific way to query elements on the page, you
+can define a custom strategy used by the testing library.
+
+For that register your strategy once:
+
+```js
+configure({
+  queryElement: (element, query) => element.querySelector(query),
+  queryAllElements: (element, query) => element.querySelectorAll(query),
+})
+```
+
+The interface of the functions will take two arguments, `element` and `query`,
+which the wrapper can use. The expected return value is a single element (for
+queryElement) or a list of elements (for queryAllElements).
+
+This can be used to allow to extend queries to reach into the shadow dom or
+iframes. An example using the
+[query-selector-shadow-dom](https://github.com/Georgegriff/query-selector-shadow-dom)
+library would look like the following:
+
+```js
+import {
+  querySelectorAllDeep,
+  querySelectorDeep,
+} from 'query-selector-shadow-dom'
+configure({
+  queryElement: querySelectorDeep,
+  queryAllElements: querySelectorAllDeep,
+})
+```
+
+Default to `element.querySelector` and `element.querySelectorAll`.