Update readme files (#3098)

* Update readme files

* Update README.md

CONTRIBUTING.md

@@ -41,10 +41,6 @@ Contributions made by corporations are covered by a different agreement than the
 
 ## How to run DartPad locally
 
-To run the server, see https://github.com/dart-lang/dart-pad/blob/main/pkgs/dart_services/README.md.
+To run the server, see the [dart_services readme](https://github.com/dart-lang/dart-pad/blob/main/pkgs/dart_services/README.md).
 
-To run the front-end, from the `pkg/dart-pad` directory, run:
-
-```
-dart tool/grind.dart serve
-```
+To run the front-end, see the [dartpad_ui readme](https://github.com/dart-lang/dart-pad/blob/main/pkgs/dartpad_ui/README.md).

pkgs/dartpad_ui/README.md

@@ -1,15 +1,6 @@
-## SketchPad
+# DartPad UI
 
-An experimental redux of the DartPad UI.
-
-## Status?
-
-This is an experimental re-imagining of the DartPad UI. Goals include:
-
-- re-writing the front-end in Flutter Web
-- having a simple, visually interesting UI
-- keeping the number of use cases small; for example, currently only Dart
-  snippets and Flutter web apps are supported
+The main DartPad web UI implemented using [Flutter Web](https://flutter.dev/multi-platform/web).
 
 ## How to run