Add warning page for RTR deprecation (#6632)

* Add warning page for RTR deprecation

* shorten path

* Add link for RN testing library

Author: jackpope

src/content/warnings/react-test-renderer.md

@@ -0,0 +1,7 @@
+---
+title: react-test-renderer Deprecation Warning
+---
+
+react-test-renderer is deprecated. A warning will fire whenever calling ReactTestRenderer.create() or ReactShallowRender.render(). The react-test-renderer package will remain available on NPM but will not be maintained and may break with new React features or changes to React's internals.
+
+The React Team recommends migrating your tests to [@testing-library/react](https://testing-library.com/docs/react-testing-library/intro/) or [@testing-library/react-native](https://callstack.github.io/react-native-testing-library/docs/getting-started) for a modern and well supported testing experience.