Document exporting individual panels from a dashboard (CSV report from a Discover session panel)

## Summary

Dashboard export documentation covers **dashboard-level** exports but not **per-panel** exports. In particular, generating a **CSV report from a Discover session panel** is undocumented, even though it's a distinct action from the visualization-data "Download CSV" that the docs already describe.

[Share and export dashboards](https://www.elastic.co/docs/explore-analyze/dashboards/sharing) lists three export paths:
- Export as PDF or PNG (whole dashboard)
- Download visualization data as CSV (`#download-csv`) — inline download of the data behind a **Lens/chart/table visualization**
- Export JSON (dashboard definition)

There is no entry for exporting the contents of an individual **Discover session panel** as a CSV report. That panel action is a separate mechanism: it uses the reporting system (immediate or scheduled), it's labeled **Generate CSV report** in the UI (not **Download CSV**), and it works on saved-search / Discover session panels rather than visualizations.

## Why this needs docs

- A dashboard user with a Discover session panel who wants a CSV has no documented path. `sharing.md#download-csv` explicitly scopes itself to visualizations ("chart and table visualizations that expose tabular data, typically those created with **Lens**"), so it doesn't cover them.
- The two mechanisms are easy to conflate because both surface CSV. Clarifying the distinction (visualization data download vs. Discover session panel CSV report) would help.
- [`save-open-search.md`](https://www.elastic.co/docs/explore-analyze/discover/save-open-search) "Manage Discover session panels on a dashboard" documents editing and tab selection for these panels but says nothing about exporting them.

## Suggested scope

- Add a "Generate a CSV report from a Discover session panel" entry/section to [Share and export dashboards](https://www.elastic.co/docs/explore-analyze/dashboards/sharing), or cross-reference it from there, covering immediate and scheduled reports.
- Clarify the difference between **Download CSV** (visualization data) and **Generate CSV report** (Discover session panel, reporting path).
- Verify UI labels at HEAD before drafting: the panel action is `reporting.share.panelAction.generateCsvPanelTitle` → **Generate CSV report** (confirmed in `elastic/kibana` at time of filing).

## Context / related

- Split out from #7632 / elastic/docs-content#7673, which documents that CSV reports of ES|QL Discover sessions and Discover session panels apply the currently selected variable control values. That PR intentionally scoped the dashboard-panel export documentation out, since it's a broader pre-existing gap.
- Implementation reference for the panel action: `src/platform/packages/private/kbn-reporting/get_csv_panel_actions/` in `elastic/kibana`.