feat: add uipath-ai-center skill for ML models and Document Understanding

[1aifanatic]

Summary

• Adds a new skills/uipath-ai-center/ skill covering UiPath AI Center and Document Understanding
• Covers the full Document Understanding pipeline: Digitize → Classify → Extract → Validate → Export
• Includes out-of-box ML extractors (Invoice, Receipt, PO, ID card), custom model training workflow, and ML Skill deployment via AI Center
• Documents AI Center ML Skill deployment steps (upload package, create skill, get endpoint URL) and how to call ML Skills from coded and XAML workflows
• Covers human-in-the-loop validation patterns using Action Center tasks and Orchestrator queues
• Includes minimum training sample recommendations, model monitoring metrics, and a common pitfalls table

Motivation

Document Understanding and AI Center are among the highest-value UiPath features for business automation, yet they are not covered by any existing skill. This gap means AI agents cannot guide users through setting up intelligent document processing pipelines without this skill.

Test plan

• Verify SKILL.md frontmatter format matches existing skills
• Verify UiPath.DocumentUnderstanding.ML.Activities package name is correct
• Verify AI Center REST API endpoint patterns are consistent with platform skill patterns
• Verify cross-references to other skills use correct relative paths