feat(c8ctl): add alpha communication and customer context

[Copilot]

Description

Addresses PM review feedback: alpha status was under-communicated across c8ctl docs, and customer context (audience, supported environments) was missing.

• Alpha admonitions: Replaced :::note with :::caution Alpha feature on getting-started.md (links to alpha features policy and GitHub repo). Added short :::caution admonitions to cluster-inspection.md, development-workflows.md, and plugins.md.
• Audience context: Added who-this-is-for (developers needing scriptable cluster access during dev/testing) and supported environments (SaaS + Self-Managed) to getting-started.md.
• Card indicator: Appended "(Alpha)" to the c8ctl CLI card description in _apitools-card-data.js.

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 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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