Document how to hide images in Element

[andybalaam]

As discussed within the Trust and Safety Research and Documentation Working Group.

✔️ 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.
• Let us know if you are contributing in a specific role, such as on behalf of an organisation or team, for example.
• Let us know if your PR is time-sensitive in any way.
• Mention any issues related to the PR. Use GitHub keywords as appropriate.
• Your individual commits or pull request is signed off.

Signed-off-by: Andy Balaam [email protected]

[HarHarLinks]

Relates to #2685