Update sql-database-auditing-get-started.md

Removing preview text - Azure SQL DB Auditing became GA
Adding images for updating sql-database-auditing-get-started article
Merge branch 'master' of https://github.com/Azure/azure-content
Updates to auditing getting started

1. Up to date images
2. Some text refinements
3. New section: "Practices for usage in production at the button
Refinements after feedback

Test refinements
Images for security enabled access and key rotation
Referring also to PowerShell
Adding REST API Reference
Merge pull request #2301 from nadavhelfman/master

Updating "Auditing Getting Started"