Update variable mediator docs

[chathuranga-jayanath-99]

Current Limitation

It is possible to define variables in the script of a script mediator. It is not included in the variable mediator docs.

var sum = 5;
mc.setVariable('result_str', sum);

Suggested Improvement

Update the doc with above details.

Version

No response

[SanojPunchihewa]

We need to update the doc at https://mi.docs.wso2.com/en/4.4.0/reference/mediators/script-mediator/