Basic documentation about the extension registration system is at https://www.mediawiki.org/wiki/Extension_registration
The convertExtensionToRegistration.php script can be used to create an extension.json file, though it may not work perfectly and could trigger errors. Use the validateRegistrationFile.php script to make sure your script follows the correct schema.
The PHP entry point will need to be kept and there will be duplication of information until Wikimedia config is updated.
Please add me (Legoktm) as a reviewer to conversion patches and feel free to ask me if something is unclear, some documentation is missing, etc.
Current progress: https://tools.wmflabs.org/extreg-wos/
Pending open tasks: https://phabricator.wikimedia.org/maniphest/query/JZZZq2aG_rR3/#R
You can use this form to add new subtasks to this one.