Add example of versioned source model metadata

[FHomewood]

Add example of versioned source model metadata.

This change is in response to a post in the ADV slack written by user an. As model versioning in dbt becomes more popular it is a good idea to add examples of this in the docs as it might not be 100% apparent how this works behind the scenes.

Could be a good feature to add custom support for this to the base package but since this works I don't think it's urgent. Can happily add an gh-issue to the main repository if it's something you guys might be interested in adding down the line?

Changelog

Changes

• "ref style" source model metadata is now in a tabbed code block.

Adds

• "versioned model" tab to the ref style metadata example.