Needed to close#8499.

To include

• What is a guard
• Why you would use a guard
• Guards as classes (event subscribers)
• Guards as ExpressionLanguage expressions (guard: in YAML config)
• Available parameters
  • subject etc

Existing online resources

https://codereviewvideos.com/course/symfony-workflow-component-tutorial/video/workflow-guards-part-1 has an excellent series (written for Symfony 3.3)