docs: build Maestro BPMN authoring guide

[gozhang2]

Summary

• Add the uipath-maestro-bpmn planning charter and sanitized frontend XML contract
• Add Flow-style Author guidance centered on two-pass BPMN authoring: skeleton first, operator shape confirmation, then model-owned UiPath XML fill
• Document greenfield, brownfield, editing operations, variables, bindings, expressions, validation, and the Integration Service CLI enrichment boundary

Verification

• bash skills/uipath-maestro-bpmn/.maintenance/check-all.sh
• run_workspace_checks(kinds="auto") completed: pytest passed; repo-wide Python fmt/lint failures remain in unrelated tests/ files
• run_workspace_checks(check_kinds=["fmt","lint","typecheck","test","test_web"]) completed: pytest passed, typecheck/test_web skipped by plan; repo-wide Python fmt/lint failures remain in unrelated tests/ files

Review notes

Draft PR for human review before merge. Public docs use synthetic guidance only and keep Integration Service connection details, generated schemas, and package metadata in the CLI-owned boundary.

[github-actions]

⚠️ Smoke test coverage gap — the following changed skills have no tests under tests/tasks/:

• uipath-maestro-bpmn

Consider adding smoke tests before merging.