Replace ECOSYSTEM.md with Foundation site link (#57949)

Summary:
Pull Request resolved: #57949

Changelog: [Internal]

Reviewed By: vzaidman

Differential Revision: D115886097

fbshipit-source-id: 6ce920da1e2de8a7dbd26cb824abdbbc7186afe6

Author: huntie

CONTRIBUTING.md

@@ -1,7 +1,6 @@
 # Contributing to React Native
 
 Thank you for your interest in contributing to React Native! From commenting on and triaging issues, to reviewing and sending Pull Requests, all contributions are welcome.
-We aim to build a vibrant and inclusive [ecosystem of partners, core contributors, and community](ECOSYSTEM.md) that goes beyond the main React Native GitHub repository.
 
 To learn more about how to contribute check out the Contributing section on the React Native website:
 * https://reactnative.dev/contributing/overview

ECOSYSTEM.md

@@ -45,7 +45,7 @@ in JavaScript, rendered with native code.
 - **Developer Velocity.** See local changes in seconds. Changes to JavaScript code are applied with Fast Refresh, without rebuilding the native app.
 - **Extend it yourself.** Native Modules let you call platform code directly from JavaScript, synchronously and type-safe — or reach for [thousands of existing libraries](https://reactnative.directory/).
 
-React Native is developed and supported by many companies and individual core contributors. Find out more in our [ecosystem overview](https://github.com/react/react-native/blob/HEAD/ECOSYSTEM.md).
+React Native is developed and supported by many companies and individual core contributors. Find out more on the [React Foundation website](https://react.foundation/).
 
 ## Building your first React Native app