Document the "Export agents to CSV" bulk action in Fleet

## Summary

The Fleet Agents list supports exporting agents to a CSV file via a bulk action, but this feature is not documented in the Fleet docs (`/reference/fleet`). We should add user-facing documentation for it.

The feature was implemented in [elastic/kibana#196635](https://github.com/elastic/kibana/pull/196635). It is available in 8.18 and 9.0+.

## What the feature does

From the **Fleet > Agents** tab, users can export the agent list to a CSV file:
- It's a **bulk action** ("Export X agents as CSV") available after selecting agents (or selecting all).
- The export respects the **filters** and **sort order** currently applied to the agent list.
- It's built on the **Kibana Reporting framework**: a toast notification links to the generated report, which can be downloaded from **Reporting**.
- It requires [reporting privileges](/deploy-manage/kibana-reporting-configuration.md#grant-user-access).

## Proposed changes

- Add documentation for the export action, most likely on the Fleet Agents management page (`reference/fleet/manage-agents.md`), where the other bulk actions (unenroll, upgrade, migrate, tag) are listed.
- Include a note about the required reporting privileges.
- Use the existing Security docs as a model—see the "CSV export" entry in `solutions/security/detect-and-alert/manage-detection-alerts.md`.

## References

- Implementation PR: [elastic/kibana#196635](https://github.com/elastic/kibana/pull/196635)
- Related bug fix: [elastic/kibana#225050](https://github.com/elastic/kibana/pull/225050) ("Fixes export CSV in the Agent list in Data ingestion and Fleet")

Start with reference/fleet/manage-agents.md, where the other Fleet Agents bulk actions are documented, and compare the CSV export entry in solutions/security/detect-and-alert/manage-detection-alerts.md. Use the linked implementation details to confirm the supported versions, reporting privilege requirement, filters, sort order, and Reporting download flow. Done means the Fleet documentation clearly describes the export action and its access requirement.

Written by the indexing model from the issue text.