Add description to _cluster/stats ccs section

[smalyshev]

Document ccs section in _cluster/stats response.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
cluster.stats	🔴	27/27	0/27

You can validate these APIs yourself by using the make validate target.

[Copilot]

Pull Request Overview

This PR documents the cross-cluster search (CCS) section in the _cluster/stats response. The key changes include:

• Adding detailed documentation and new type classes/interfaces for cross-cluster stats (CCSStats, RemoteClusterInfo, CCSUsageStats, etc.) in the TypeScript definitions.
• Updating the StatsResponseBase in ClusterStatsResponse.ts to include the new ccs property.
• Propagating similar changes to the generated output types in output/typescript/types.ts.

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
specification/cluster/stats/types.ts	Added new CCS types and documentation for cross-cluster stats.
specification/cluster/stats/ClusterStatsResponse.ts	Updated StatsResponseBase to include the ccs field.
output/typescript/types.ts	Added corresponding interfaces for CCS stats and remote cluster info.

Files not reviewed (1)

• output/openapi/elasticsearch-openapi.json: Language not supported