chore(markdown): iframe fallbacks for the quickstart in both Markdown twins

Part of the Markdown for Agents Phase 1 epic (see #5652). kestra.io has Cloudflare Markdown for Agents enabled: `Accept: text/markdown` converts the SSR HTML at the edge, so defects in the HTML end up in what AI agents read. Cloudflare strips `nav`/`header`/`footer`/scripts/styles/inline SVG, ignores `aria-hidden` and empty `alt`, keeps `.visually-hidden` text and `display:none` content, and renders `

` as separate lines (all verified on production).

## Problem
Two documents exist for the quickstart. `/docs/quickstart.md` (served by `src/pages/docs/[...docsPath].md.ts`) returns the raw body with the literal YouTube and Arcade `` HTML. `/docs/quickstart` requested as Markdown is Cloudflare-converted and the iframes vanish entirely. Same shape in the blog, resources and whitepaper `.md.ts` routes.

## Scope
- Small helper shared by the five `.md.ts` routes that rewrites `<iframe src="…" title="…">` to a Markdown link with the title.
- Add a visible fallback link next to the video and Arcade embeds in the docs HTML (for example under the `video-container`), so the converted twin also carries the URLs.

## Done when
- Both `/docs/quickstart.md` and converted `/docs/quickstart` mention the YouTube and Arcade URLs.

Start with src/pages/docs/[...docsPath].md.ts and locate the corresponding five .md.ts routes that serve documentation, blog, resources, and whitepaper content. Trace the quickstart video-container embeds and the shared iframe-rewriting path, then verify both /docs/quickstart.md and converted /docs/quickstart mention the YouTube and Arcade URLs.

Written by the indexing model from the issue text.