Add proper instructions to use already created data source for a data service

[AmilaSamith]

Current Limitation

Current documentation doesn't provide clear steps in using already created data source in project for creating a new data services with VS Code extension.

Suggested Improvement

Provide a NOTE on how to select and use a existing carbon data source when creating a new data source. Better if we can add screenshot to make steps more clear like below.

Version

4.3.0