[DOCS] Add fan-in, fan-out section to docs

[TomFern]

Documentation Location

https://docs.semaphore.io/using-semaphore/workflows#modeling-complex-workflows

The old docs had documentation on the "Fan-out, Fan-in" workflow type (docs/essentials/modeling-complex-workflows.md) We need to add it back to the new docs. We also have a new YouTube video we can embed in the docs.

Type of Documentation Issue

• Missing documentation
• Incorrect information
• Unclear explanation
• Outdated content
• Broken link
• Code example needs update
• Other (please describe)

Current State

Fan-out fan-in does not exist in the docs

Requested Changes

Add back fan-out fan-in

Suggested Content

Additional Information

Affects Version

• Cloud Semaphore
• Community Edition (specify version): v1.4
• Enterprise Edition (specify version): v1.5

[TomFern]

Merged into docs