docs: Add documentation for elasticsearch_shards and elasticsearch_replicas

What does this MR do and why?

Documents the existing but undocumented elasticsearch_shards and elasticsearch_replicas parameters in the Application Settings API.

These parameters already support two update modes:

1. Scalar values - Set the same number of shards/replicas for all Elasticsearch indices
2. Object values - Set different values per index using index alias names as keys

This documentation addition:

• Adds elasticsearch_shards and elasticsearch_replicas to the API parameter table
• Adds a new "Elasticsearch index settings" section with usage examples for both modes
• Documents the requirement that both parameters must be provided together when using per-index values

This is a documentation-only MR that complements !216396 which adds the elasticsearch_index_settings GET endpoint.

🛠️ with ❤️ at Siemens

References

• Related issue: #571263
• Related MR (GET endpoint): !216396
• Implementation code: ee/app/services/ee/application_settings/update_service.rb#L77-99

Screenshots or screen recordings

Not applicable - documentation only changes.

How to set up and validate locally

• Review the documentation changes in doc/api/settings.md
• Verify the curl examples are correct by testing against a local GDK instance:

  # Update all indices at once
  curl --request PUT \
    --header "PRIVATE-TOKEN: <token>" \
    --header "Content-Type: application/json" \
    --url "http://gdk.local:3000/api/v4/application/settings" \
    --data '{"elasticsearch_shards": 5, "elasticsearch_replicas": 1}'
  
  # Update specific indices individually
  curl --request PUT \
    --header "PRIVATE-TOKEN: <token>" \
    --header "Content-Type: application/json" \
    --url "http://gdk.local:3000/api/v4/application/settings" \
    --data '{"elasticsearch_shards": {"gitlab-development": 5}, "elasticsearch_replicas": {"gitlab-development": 1}}'

• Run the docs linter: vale doc/api/settings.md

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug - N/A (docs only)
• Tested in all supported browsers - N/A (docs only)
• Conforms to the code review guidelines
• Conforms to the style guides
• Conforms to the javascript style guides - N/A
• Conforms to the database guides - N/A
• Conforms to the merge request performance guidelines - N/A
• Documentation follows GitLab docs style guide