docs: add workflow concurrency limits to platform limits page

[k1LoW]

Summary

• Document the two new platform-level workflow concurrency limits: workspace-wide (50) and per-workflow (20)

Changes

• Add workspace concurrent executions limit and per-workflow concurrent executions limit to the Service Limits table
• Add "Workflow Concurrency Limits" section explaining how the limits work and their behavior
• Add workflow concurrency best practice to the Best Practices section

[Copilot]

Pull request overview

Documents newly introduced workflow execution concurrency limits on the Tailor Platform, clarifying how they’re enforced and what users should expect when limits are reached.

Changes:

• Expanded the Service Limits table to include workspace-wide and per-workflow workflow execution concurrency caps.
• Added a Workflow Concurrency Limits section describing enforcement and scheduling behavior (PENDING → RUNNING).
• Updated Best Practices to include guidance for designing around concurrency queuing.

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