Refactor cross-cluster search docs

Page: https://www.elastic.co/docs/explore-analyze/cross-cluster-search

## Problem
- The current cross-cluster search (CCS) page mixes multiple content types: overview, setup, tutorial, API reference, troubleshooting, and upgrade planning. This may have made sense historically as a single Elasticsearch reference module, but it conflicts with our content-type expectations. The size of the page is overwhelming for any user to manage and it makes finding things more difficult (content is not easy to scan).
- Consequences: readers who want to run a query must wade through async-polling JSON and implementation details; readers planning upgrades may never reach the compatibility table at the bottom; maintainability and discoverability suffer.

## Proposed solution

The docs could be split into an overview hub, task how-tos, reference, and possibly troubleshooting. The setup vs solution workflows must be easier to find and focus on.

I ran into this page as part of https://github.com/elastic/docs-content/pull/6934 while moving the CCS-specific privileges info and noticed this, so wanted to log a backlog item for it.

Start with the cross-cluster search page at https://www.elastic.co/docs/explore-analyze/cross-cluster-search and inventory its overview, setup, tutorial, API reference, troubleshooting, and upgrade-planning content. Define a split into an overview hub, task how-tos, reference, and possibly troubleshooting, with setup and solution workflows easier to find; done means the content is separated and discoverable without the current scanning burden.

Written by the indexing model from the issue text.