Document user API key authentication for cross-cluster search requests

The [CCS](https://www.elastic.co/docs/explore-analyze/cross-cluster-search) page explains how to configure privileges for CCS, but all examples assume username/password users:

* Search examples in [CCS examples](https://www.elastic.co/docs/explore-analyze/cross-cluster-search#ccs-example) show no authentication header
* [after https://github.com/elastic/docs-content/pull/6934 is merged]: Role creation uses `POST /_security/role/` and access is granted via `POST /_security/user/` with a password.

https://github.com/elastic/docs-content/pull/6934 adds the **API key authentication** section that describes the remote cluster security model (cross-cluster API key for connecting clusters), not how an end user or application authenticates CCS search requests. Readers may incorrectly assume CCS requires user/password auth, or confuse cross-cluster API keys with user REST API keys.

It would be useful to:
* clarify terminology to distinguish cross-cluster API key from REST user API key (request authentication)
* add an example for creating a user API key for CCS (with remote indices and optionally local ones in role descriptors)
* add an example of running a CCS request with the created key
* explain that remote access is the intersection of the API key's privileges and the key owner's privileges

Useful resources:
* https://www.elastic.co/docs/deploy-manage/api-keys/elasticsearch-api-keys
* https://www.elastic.co/docs/troubleshoot/elasticsearch/remote-clusters

Start with the CCS page and its linked CCS examples, then review the API keys and remote-clusters resources listed in the issue. Add terminology distinguishing cross-cluster API keys from user REST API keys, a user API key creation example, and a CCS request using it. Done means the examples cover remote and optional local indices and explain the intersection of API-key and owner privileges.

Written by the indexing model from the issue text.