docs(cloud): Rewrite CLI reference introduction and rename category to CLI

[developerjamiu]

PR 10 of the Cloud IA rollout (#568). Scaffolding only; substantive command-by-command writing is PR 11 + PR 12.

Summary

• Rewrite `cloud_docs/reference/cli/introduction/introduction.md` from a tutorial-shaped walkthrough (install → login → launch → deploy → show) into a meta-template-shaped reference landing. Sections: opener, install + authenticate (link-only), command syntax shape, global options summary, environment variables summary, where each topic area's concept page lives, and how the reference is organized (hand-written intros + auto-generated subcommand listings). Tutorial content has its own home now ("Deploy your first app") so the intro no longer duplicates it.
• Rename the CLI category label from "CLI reference" to "CLI" in `category.json`. URL path `/cloud/reference/cli/*` and sidebar position (9 under Reference) are unchanged.

The original plan considered promoting CLI reference to a top-level sidebar section per IA §4, but the user opted to keep it nested under Reference for now. The label rename is the visible change.

Test plan

• `npm run build` passes in CI.
• Open the new `/cloud/reference/cli/introduction/introduction` page and read top to bottom; every link resolves.
• Sidebar shows Reference → CLI (with the renamed label) and the existing position.
• Inbound links to `/cloud/reference/cli/*` from other pages still resolve (no URL changes).