add Matrix Live with guidance about room version 12

[HarHarLinks]

🎩 Website & Content WG

✔️ Checklist

• Check for common mistakes:
  • Wrap plain URLs in <> to linkify them (learn more).
  • Use the right level of headings: The page title will use a level 1 headings, so your headings should use level 2 and below.
  • Use internal links: when linking to another page on https://matrix.org, use the Zola [label](@/target.md) syntax.
• For blog posts:
  • Verify the date and post ordering on the /blog page, especially for multiple posts on the same day. Prefer UTC format, e.g. 2025-12-01T14:00:00Z for Dec 1st, 2025, 2pm UTC.
  • Set the correct author and category. Browse existing ones at https://matrix.org/author/ and https://matrix.org/category/ to match them.
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[HarHarLinks]

I considered using the youtube embed alternatively. Didn't choose it mostly because I'm not sure about the support for including the time stamp. While the whole video is somewhat relevant, my intention is to skip to the part with explicit guidance since this is a guide.