docs(cloud): Establish Reference section by moving and renaming the three existing pages

[developerjamiu]

PR 13 of the Cloud IA rollout. Moves the three existing reference pages out of reference/deployment/ to top-level Reference, renames each to the IA target title, adds redirects for every old URL, and scopes the Concepts category icon so it stops cascading to every child page.

Changes

• reference/deployment/handling-private-dependencies.md to reference/private-dependencies.md, title "Handling private dependencies" to "Private dependencies".
• reference/deployment/dart-sdk-versions.md to reference/dart-sdk-versions.md, title "Dart SDK version handling" to "Dart SDK versions".
• reference/project-id.md to reference/project-id-rules.md, title "Project identifier" to "Project identifier rules".
• Each renamed page got a description in frontmatter so the page surfaces well in SEO and link previews.
• Empty reference/deployment/ folder removed.
• cli/_category_.json position set to 4. Reference now reads: Dart SDK versions, Private dependencies, Project identifier rules, CLI. Putting CLI last is curated (it is a complex folder; keeping the smaller reference pages visible first); switch to alphabetical if you prefer.
• Three redirects in docusaurus.config.js for the old URLs.
• Cross-doc references updated in the Deployments concept, the Recover from a failed deploy guide, and the Related section inside dart-sdk-versions.md.
• sidebar-icon-overview CSS selector was using a descendant match and was applying the compass icon to every Concepts child page. Scoped it to direct child (>) so the icon now only appears on the Concepts category header. The framework's how-it-works.md page that also uses this class still picks up the icon via the leaf branch of the selector.

Test plan

• npm run build passes
• Reference sidebar renders as: Dart SDK versions, Private dependencies, Project identifier rules, CLI
• Open /cloud/reference/deployment/handling-private-dependencies, /cloud/reference/deployment/dart-sdk-versions, /cloud/reference/project-id and confirm each redirects to the new path
• Concepts sidebar shows the compass icon only on the Concepts category header, not on Deployments, Logs, etc.
• How it works page in framework docs still shows the compass icon
• Cross-doc links in the Deployments concept and Recover guide resolve