fix(environments): correct search engine visibility scoping

[catplat]

Summary

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

• Corrects a factual inaccuracy: the Hide from search engines setting is disabled for any environment with a custom domain, not just production environments.
• Rewrites the intro to explain why hiding environments from search engines matters.
• Clarifies that the behavior applies to all environments, not just production.

Related Issue

Closes #5529

Changes

Both sites/platform/src/environments/search-engine-visibility.md and sites/upsun/src/environments/search-engine-visibility.md:

• Description: Updated to focus on protecting SEO health.
• Intro: Rewritten to explain the SEO risk and clarify default behavior applies to all environments without a custom domain.
• Note added: If multiple environments have custom domains, only one should be visible to search engines.
• Lead-in: "To change your production environment's visibility" → "To change an environment's visibility"
• Tab order: Console first, CLI second.
• Console steps: "Select a project" → "Select the project you want to update"; "select the production environment" → "select the environment whose visibility you want to change"; removed "near the upper right of the page" and "On the General tab".
• CLI: Removed "To not restrict indexers on your production environment" lead-in; PRODUCTION_ENVIRONMENT_NAME → ENVIRONMENT_NAME.
• Caveat: Moved into a {{< note >}} block.
• Section heading: "How it's done" → "How it works".
• How it works: Simplified; removed redundant paragraph.
• Section heading: "Alternative method" → "Alternative method: Use a robots.txt file".
• Last sentence: Rewritten for clarity — makes explicit that robots.txt takes precedence independently of the header setting.

Testing

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

[catplat]

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