Docs: Add warning about promoting secondary with selective sync

What does this MR do?

This MR adds critical documentation warnings about the risk of data loss when promoting a Geo secondary site that has selective synchronization enabled.

When selective synchronization is configured on a secondary site, only a subset of data is replicated (specific groups, shards, or organizations). Currently:

1. There is no validation in the promotion code (ee/lib/gitlab/geo/geo_tasks.rb) to check if selective sync is enabled
2. There is no warning in the documentation about the permanent data loss that occurs when promoting such a secondary
3. Administrators may unknowingly promote a secondary with selective sync, resulting in permanent loss of all non-replicated data

This MR adds:

1. A new section in doc/administration/geo/replication/selective_synchronization.md titled "Promoting a secondary site with selective synchronization enabled" that:
   • Clearly warns about permanent data loss
   • Explains what data will be lost
   • Provides recommendations for safe promotion
   • Offers guidance for emergency scenarios

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
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  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.