Celesta 7.x API documentation rework

[Ivan Ponomarev]

Hi everyone! happy new year!

Celesta 7.x API documentation was substantially reworked and is available here: https://courseorchestra.github.io/celesta/apidocs/

All the Javadoc was translated to English, the Javadoc coverage of classes and methods has been increased. For now, only occasional comments in Russian remain in Celesta code, and, of course, the User Guide is still in Russian, which we are going to trnaslate this year.

Thank you Pavel for the great job!

Regards,

Ivan