doc: Add domain and tenant implications to k8s config

[vlasebian]

Summary

Closes #1473

Changes

• Add a "Implications of choosing the domain and initial tenant" section to the Kubernetes generic configuration page.
• Explain how global.domain, global.deployment.initialTenant.tenantID and global.tenancy.defaultID affect instance URLs and TLS certificate requirements.
• Include concrete configuration examples for single-tenant and multi-tenant deployments.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Run Locally: Verified that the docs build using make server, posted screenshots, verified external links. Test with HUGO_PARAMS_SEARCH_ENABLED=true if style changes will affect the search bar.
• New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.

[vlasebian]

cc: @mfilipan