Document looping concepts

[markoa]

Looping is a core workflow concept but is currently undocumented. We need a dedicated docs section covering:

• How the loop component works
• Supported loop types (e.g. iterate over array, condition-based) (foreach component)
• Result accumulation across iterations
• Iteration limits and what happens when they are reached
• Nesting loops
• Common patterns and examples (e.g. fan-out, retry loop, batch processing)