You can find the general documentation at https://github.com/CiviMRF/documentation.

The CurlAuthX connector

In the latest versions of CiviCRM, the endpoint https://my-civi.org/sites/all/modules/civicrm/extern/rest.php is not supported anymore. It is replaced by a new extension AuthX that provides a lot of different authentication options. The CurlAuthX connector uses just one of them.

The AuthX does not have a graphical interface (yet). You can configure it with the cv utility.

• enable the new authx extension cv ext:enable authx
• choose the authentication method cv ev 'Civi::settings()->set("authx_xheader_cred",["api_key"]);'
• make sitekey mandatory cv ev 'Civi::settings()->set("authx_guards",["site_key"]);'
• refresh the cache cv flush