docs: Add global user task listeners documentation

[Copilot]

Documents the Global User Task Listeners feature (camunda/camunda#43168), which supports cluster-wide user task lifecycle listeners configurable via the Orchestration Cluster API, Admin UI, and Unified Configuration.

Description

New: docs/components/concepts/global-user-task-listeners.md

Core concept page covering:

• What they are: cluster-wide user task listeners defined once, applied across all processes without BPMN changes
• Management approaches table: Admin UI (no restart), Orchestration Cluster API (no restart), Unified Configuration (restart required); API/UI and config-file listeners coexist independently
• Unified Configuration: full property reference, validation rules, YAML + env var examples, apply-changes guidance
• Execution order: config-file listeners → model-level listeners → API/UI listeners; after-non-global flag respected per set
• Supported features: delegates to user-task-listeners.md for blocking behavior, corrections, deny, incident recovery
• Limitations: no tenant-specific config; config-file changes require restart; API/UI changes take effect immediately

Note: API spec files (.api.mdx, sidebar.ts) and the OpenAPI YAML are auto-generated from server code per camunda/camunda docs and are not committed here.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support. (add bug or support label)
• This is already available but undocumented and should be released within a week. (add available & undocumented label)
• This is on a specific schedule and the assignee will coordinate a release with the Documentation team. (create draft PR and/or add hold label)
• This is part of a scheduled alpha or minor. (add alpha or minor label)
• There is no urgency with this change (add low prio label)

PR Checklist

• The commit history of this PR is cleaned up, using {type}(scope): {description} commit message(s)

• My changes are for an upcoming minor release and are in the /docs directory (version 8.9).

• My changes are for an already released minor and are in a /versioned_docs directory.

• I included my new page in the sidebar file(s).

• I added a redirect for a renamed or deleted page to the .htaccess file.

• I added a DRI, team, or delegate as a reviewer for technical accuracy and grammar/style:

  • Engineering team review
  • Technical writer review via @camunda/tech-writers unless working with an embedded writer.

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