docs: add data app visualizations guide

[almeidabbm]

Summary

Adds user-facing documentation for data app visualizations - AI-generated, reusable custom chart types built on the data apps infrastructure.

• New guide page guides/data-apps/visualizations covering:
  • Creating a visualization from the Data app visualization template card in the app generator (prompt -> renderer + declared field schema)
  • Testing it with real data via the interactive "Visualization ready" test panel (ephemeral, no saved chart)
  • Using it as a chart type in the Explorer: searchable visualization dropdown + field mapping (required fields can't be cleared)
  • Saving and dashboards: saved charts remember the visualization + mapping; dashboard filters and date zoom apply; one visualization reused across many charts
  • Permissions cross-link to the existing data apps permissions section
• Linked from the data apps landing page (new "Visualizations" section)
• Short entry on the chart types overview alongside Custom chart, noting it requires data apps to be enabled
• Registered in the nav (docs.json)

Screenshots are left as TODO Frame placeholders for: template picker card, test panel, Explorer chart-type picker + field mapping, and a dashboard tile.

Display config options (colors, toggles) are intentionally not documented - that UI has not shipped yet.

Verification

• mint broken-links passes; the only flagged links are the 4 intentional TODO image placeholders.
• docs.json validates as JSON.

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Status

⚠️ 4 broken links need manual attention

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🔗 Broken Links (Manual Fix Required)

Click to view details

   📄 guides/data-apps/visualizations.mdx:24 → /images/TODO-data-app-viz-template-card.png
   📄 guides/data-apps/visualizations.mdx:42 → /images/TODO-data-app-viz-test-panel.png
   📄 guides/data-apps/visualizations.mdx:59 → /images/TODO-data-app-viz-explorer-config.png
   📄 guides/data-apps/visualizations.mdx:71 → /images/TODO-data-app-viz-dashboard-tile.png

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