docs: add iframe embed section for external sites to rich media page

[devin-ai-integration]

Summary

Adds a brief "Embed external sites" subsection to the Rich media in Markdown page, right after the existing video embed section. Covers:

• Basic <iframe> syntax for embedding external web pages
• A note about X-Frame-Options / CSP restrictions
• A cross-reference to the Embedded mode page for embedding Fern docs in other applications

This is intentionally brief since iframes are standard HTML, not a Fern-specific feature. The page already documents <iframe> for PDFs and video embeds — this fills the gap for general external site embeds.

Review & Testing Checklist for Human

• Verify the new section renders correctly on the preview deployment
• Confirm the cross-reference link to Embedded mode resolves

Notes

Kept minimal per the request — just enough to make iframe support discoverable without overstating it as a Fern product feature.

Link to Devin session: https://app.devin.ai/sessions/f08e588470cf4db6bedcf4059223ed88
Requested by: @devalog

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1779495294-iframe-docs.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/writing-content/markdown-media
• learn/docs/customization/embedded-mode