[Website] Update Learning-resources page

Need to update Learning-resourses page https://beam.apache.org/documentation/resources/learning-resources/

1. In Code Examples section add a new point: Beam Playground (https://beam.apache.org/get-started/try-beam-playground/)
2. Playground section contains 2 points:
   1. Beam Playground is an interactive environment to try out Beam transforms and examples without having to install Apache Beam in your environment.
      You can try the available Apache Beam examples at Beam Playground (http://play.beam.apache.org)
   2. Learn more about how to add an Apache Beam example/test/kata into Beam Playground catalog here (https://beam.apache.org/get-started/try-beam-playground/#how-to-add-new-examples).