docs(cloud): Add Deployments concept page, redirect the old reference URL

[developerjamiu]

PR 2 of the Cloud IA rollout (#568). Establishes the Concepts section in the sidebar and ships the first Concepts page.

Changes

• NEW cloud_docs/02-concepts/01-deployments.md: Deployments concept page consolidating the old Deploy reference plus the redeploy/dry-run content that lived in the Get started tutorial. Six sections: Overview, Deploy your app, Check deployment status, Validate before deploying, Configure what's included (.scloudignore), Troubleshooting, Related.
• NEW cloud_docs/02-concepts/_category_.json: establishes the Concepts sidebar group at position 2.
• MODIFIED cloud_docs/01-getting-started/02-launch.md — removed the "Deploy updates" section; replaced with a one-line pointer to the new Deployments concept.
• DELETED cloud_docs/reference/01-deployment/01-deploying-your-application.md: content fully absorbed into the new concept page.
• MODIFIED docusaurus.config.js: added a redirect from the old URL /cloud/reference/deployment/deploying-your-application to the new /cloud/concepts/deployments so external bookmarks keep working.
• MODIFIED two reference pages (04-deployment-hooks.md, 05-dart-sdk-versions.md): updated their inbound link from the old path to the new concept page.
• MODIFIED sidebar positions (02-guides/_category_.json, reference/_category_.json): explicit positions so the new sidebar order is Get started → Concepts → Guides → Reference.

Verification

• scloud deploy --help and scloud deployment --help confirm every flag and subcommand referenced.
• Deploy lifecycle stages (Booster liftoff / Orbit acceleration / Orbital insertion / Pod commissioning) verified against real CLI output.
• npm run build passes; the redirect resolves; sidebar order is correct.

What's next

PR 3: Deploy your first app tutorial rewrite (uses this concept). PR 4-6: remaining Concepts. Full rollout sequence in the #568 tracking comment.