fix(environments): correct search engine visibility scoping

[catplat]

Summary

Updates the search engine visibility documentation to reflect actual product behavior and improve clarity.

Factual corrections

• Corrects the scoping of the Hide from search engines setting: behavior depends on project type (Grid vs Dedicated Gen 2), not just on whether a custom domain is assigned.
• Splits the Grid behavior into distinct cases by environment type and custom domain presence, including a new/existing project distinction for production environments.
• Adds a Dedicated Gen 2 section covering development environments (always hidden, enforced), staging environments (hidden by default, user can toggle), and production (always visible, enforced).
• Removes incorrect prerequisites stating that a custom domain is required to toggle the setting (not true for Dedicated Gen 2 staging environments).
• Removes the inaccurate footer "This setting can only be changed on environments with a custom domain."

Clarity improvements

• Rewrites the intro to explain why hiding environments from search engines matters.
• Updates the note about multiple-environment SEO risk to be project-type-agnostic.
• Tab order: Console first, CLI second.
• Console steps: simplified and de-scoped from production-only framing.
• CLI: removed production-only lead-in; PRODUCTION_ENVIRONMENT_NAME → ENVIRONMENT_NAME.
• Caveat moved into a {{< note >}} block.
• Section headings: "How it's done" → "How it works"; "Alternative method" → "Alternative method: Use a robots.txt file".
• Last sentence of robots.txt section rewritten for clarity.

Related Issue

Closes #5529

Changes

• sites/platform/src/environments/search-engine-visibility.md: All of the above.
• sites/upsun/src/environments/search-engine-visibility.md: Clarity improvements only (Upsun Flex handles its own scoping via a separate PR).

Testing

Prose-only changes with no structural impact on shortcodes or links.

[catplat]

@nicogommen Are you able to review/approve/merge this?