chore: fix contributing guide and chat links in README (#7223)

README.md

@@ -163,15 +163,15 @@ Are you looking for an example project to get started? Visit [start.vaadin.com](
 
 ## Questions
 
-For help and support questions, please use our [community chat](https://vaad.in/chat).
+For help and support questions, please use [Vaadin Forum](https://vaadin.com/forum/).
 
 ## Big Thanks
 
 Cross-browser Testing Platform and Open Source <3 Provided by [Sauce Labs](https://saucelabs.com).
 
 ## Contributing
 
-Read the [contributing guide](https://vaadin.com/docs/latest/contributing/overview) to learn about our development process, how to propose bugfixes and improvements, and how to test your changes to Vaadin components.
+Read the [contributing guide](https://vaadin.com/docs/latest/contributing) to learn about our development process, how to propose bugfixes and improvements, and how to test your changes to Vaadin components.
 
 ## Development