#3409 docs: Add more public classes and functions to the reference docs Pull request opened by edgarrmondragon ## Related • #2558 • Closes #2476 • Closes #2688 ## Summary by Sourcery Expand and reorganize the public API reference documentation for singer_sdk. Documentation: • Add reference pages for additional public classes, exceptions, helpers, metrics, schema sources, Singer library objects, and testing utilities in singer_sdk. • Introduce documentation redirects for re-exported schema and testing classes and for the deprecated JSONLines batch encoder path to maintain link compatibility. meltano/sdk