Add docs for operator : transformation - concatMap

[ashwin-sureshkumar]

Add docs for operator concatMap under group transformation

Below is minimum details needed

• name
• operatorType
• signature
• parameters
• short description
• Technical description
• Visual example (Marble diagram)
• Real world example (Non-framework)
• Related Operators

[luillyfe]

it will be solve by #272