More exhaustive sidekiq documentation

What does this MR do?

UPDATE: This MR was getting stale, we just revived it and I think I made the necessary changes for it to be correct.

Original description:

Being new at the company, I had a hard time digesting our sidekiq setup, as the documentation is quite dispersed and the way it operates not clearly explained anywhere. I therefore took it on me to document it better for both me and my team (cf https://gitlab.com/gitlab-org/memory-team/team-tasks/issues/16) and I was wondering if this would be useful to have on doc.gitlab as well?

Open tasks / questions

• Replace hand drawn diagram with Mermaid diagram
• Move content out of Debugging section?

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Separation of EE specific content