Review Active Record Basics Guide [ci-skip] (rails#51226)

Add more code examples and update existing examples to be more consistent across the board.
Make the connection between Active Record and Active Model clearer.
Update Intro language to be more friendly, less jargon-y, clearly explain terms like ORM.
Expand some sections like how to generate namespaces models.

Co-authored-by: Carlos Antonio da Silva <[email protected]>