[Workflow] Document Guards

[pbowyer]

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)

[noniagriconomie]

@pbowyer cc @xabbuh @javiereguiluz (from #8499) I can do a doc on this

should i create a new file or improve this sub section? https://symfony.com/doc/current/workflow/usage.html#guard-events

Thank you,

[pbowyer]

I have no strong feeling. The Workflow docs will need re-organising soon, so a separate file would be helpful.

[javiereguiluz]

@noniagriconomie if the new contents are super long, then we could create a new article. Otherwise, please update the section of the existing article. We try to avoid creating small article because they break the reading flow too much and people end up with 5 open tabs in their browsers to read 5 small articles instead of reading just one.

[javiereguiluz]

Fixed by #11433.