Change the Documentation and add playground

I only want to complain about the documentation because you don't provide a playground to test your code in place or any other information. Provided information sometimes is not understandable or less. The presence of playgraound nowdays is vital. Think over it.

Review the current Day.js documentation and its existing structure first, then identify which information is unclear and what a playground would need to support. Done means the documentation gaps are addressed and a usable playground is available for testing Day.js code in place.

Written by the indexing model from the issue text.