Restructure ES|QL task management docs to cover all query types

This issue comes from a discussion with @consulthys about the new query activity UI:

## Context

With two new features shipping in 9.4, query management and observability are no longer ES|QL-specific:

- **Query activity** (real-time): The new [Query activity UI](https://github.com/elastic/kibana/pull/253216) in Kibana surfaces running tasks for all query types (ES|QL, DSL, EQL, SQL, MSearch, Async search).
- **Query logging** (historical): The new [query logging feature](https://github.com/elastic/docs-content/pull/5039) logs all query types (DSL, ES|QL, EQL, SQL), effectively deprecating the ES|QL-specific query log.

The current [ES|QL task management page](https://www.elastic.co/docs/reference/query-languages/esql/esql-task-management) in the `elasticsearch` repo only covers ES|QL queries. It should be restructured to cover all query types, since both the real-time UI and the historical logging now work across all query languages.

## Suggested scope

- Move the task management content "up one level" from ES|QL-specific to language-agnostic
- Cover the `_tasks` API for all search task types, not just `indices:data/read/esql`
- Cross-link to:
- [Query activity](/deploy-manage/monitor/query-activity) (real-time UI in Kibana)
- [Query logging](/deploy-manage/monitor/logging-configuration/query-logs) (historical logging)
- Deprecate or redirect the ES|QL-specific task management page

## Related

- Query activity docs: https://github.com/elastic/docs-content/pull/5739
- Query logging docs: https://github.com/elastic/docs-content/pull/5039
- Query activity cross-link (elasticsearch): https://github.com/elastic/elasticsearch/pull/145397
- Original issue: https://github.com/elastic/docs-content/issues/5451