Review Fleet-related docs referenced in Kibana

Review the [Fleet-related docs referenced in Kibana](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-doc-links/src/get_doc_links.ts#L853-L901):

- Check for outdated/incorrect content
- Cross-check with existing open issues requiring improvements or fixes in these docs, create new sub-issues if necessary
- Work on the identified issues to improve the docs
