docs: document Markdown panel library save and reuse (#5695)

[florent-leborgne]

Summary

This PR addresses #5695 with the following changes:

• explore-analyze/visualize/text-panels.md: adds a new Save and reuse Markdown panels across dashboards section, scoped to stack: ga 9.4 and serverless: ga, with subsections for saving a Markdown panel to the Visualize Library, adding a saved Markdown panel from the library, and unlinking one from the library. Also wraps the existing creation steps under a new Create a Markdown panel H2 and adds a tagged sentence to the intro that points to the new section so the new capability is discoverable from the page lead.
• explore-analyze/visualize.md: extends the Markdown text row in the panels and visualizations table with a tagged sentence noting that Markdown panels can now be saved to the Visualize Library to reuse across dashboards.

Out of scope (intentionally not documented)

The issue also mentions the new Markdown REST API (POST /api/markdown/{id?}, GET /api/markdown/{id}, PUT /api/markdown/{id}, DELETE /api/markdown/{id}, GET /api/markdown) introduced by elastic/kibana#251975.

Verifying against the source, the routes use commonRouteConfig from src/platform/plugins/shared/dashboard_markdown/server/api/constants.ts, which sets access: 'internal' with the comment "This route is in development and not yet intended for public use." and the description "This functionality is in technical preview and may be changed or removed in a future release.". The endpoints are also gated behind an ?apiVersion=1 query parameter (see enableQueryVersion: true).

Given the API is explicitly internal and in technical preview, I have not added any user-facing API reference content for it. We can revisit this once the routes are promoted to a public, stable version.

Resolves

Closes #5695

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🔍 Preview links for changed docs

• explore-analyze/visualize.md
• explore-analyze/visualize/text-panels.md

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