Iterate the room upgrading guide

[HarHarLinks]

🎩 Website & Content WG in collaboration with T&S R&D WG

Signed-off-by: HarHarLinks [email protected]

Continues work on #2704, though there may be multiple iterations in this or in followup-PRs.

The prioritization/planned iterations are:

1. iteration ✅
   • what & why
   • how incl recommendations
   • client support
   • room v12 miniversion: just linking to the info that has been published. mention again that perhaps a service account show create these rooms
   • tools & scripts link dump
   • References and further reading
2. iteration
   • room v12 proper summary and recommendations
3. iteration
   • add limitations, add references from the recommendations
   • How to upgrade a room (advanced)
   • split rooms

✔️ Checklist

• Check for common mistakes:
  • Wrap plain URLs in <> to linkify them (learn more).
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• 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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• Mention any issues related to the PR. Use GitHub keywords as appropriate.
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[HarHarLinks]

TODO: rebase onto main

[TheOneWithTheBraid]

< polycule > supports upgrades using the same syntax

[TheOneWithTheBraid]

< polycule > supports jumping to tombstoned rooms

[HarHarLinks]

Consider history visibility invite